Lo and behold, fellow wayfarer! This guide was created because I was unable to find a proper guide (as in: not a link to YouTube) that showed the location of the various (unmarked) voice overs in this game that are needed to unlock the Seanchaidh achievement. Because I will, inadvertently, also be stumbling across the urn locations needed for the Tuir achievement, I decided to include these locations in the guide too (and perhaps some others like Seonaidh and Stac an Armin too...)

Even though such a screenshot-by-screenshot guide has its limitations—one being that the player generally experiences the game differently from the writer, thereby running the risk that player and writer will sometimes be out of sync—I generally prefer these old-fashioned guides over the more modern video guides on YouTube. For one, not having to watch through minutes of video footage actually saves time, and (perhaps more importantly) I can actually experience the game while playing it myself instead of having to replay a video guide. It's also infinitely more easy to see which voice overs I might've missed.

Hence, without further ado, I present thee (my first attempt at writing a Steam achievement guide) the Dear Esther: Landmark Edition screenshot-by-screenshot Seanchaidh (and Tuir) achievement guide: DELEsbsS(aT)ag for short.

This guide is divided into four sections, which follow the four different chapters of the game:

• Chapter 1: The Lighthouse: 10 voice over points
  
• Chapter 2: The Buoy: 11 voice over points
  
• Chapter 3: The Caves: 4 voice over points
  
• Chapter 4: The Beacon: 11 voice over points

Adding those up makes 36 voice over points, whereas the achievement only requires 34 to unlock. That's right, you can gimp out on one or two points and still get the achievement, although I would advise against doing this on purpose. In my run one of the voice overs failed to register so the achievement actually unlocked at the penultimate point.

To remedy this, or keep the risk to a minimum, just check every time you unlocked a voice over point whether it registered with Steam (the achievement should tell you just how many points you've already unlocked; i.e. xx/34). Sometimes they can take a while to register so wait patiently for half a minute and check again.

As of right now (25 dec 2019) DELEsbsS(aT)ag is finished and hopefully of help to those of you who are trying to get the Seanchaidh + Tuir achievements.

The Lighthouse chapter (fittingly named so for the lighthouse at the start of the game) features 10 different voice over points. And 1 urn.

The first voice over point is unmissable, it starts to play as soon as you (re)start a new game.
Interestingly, if you start the game over a couple of times—for example by drowning yourself in the ocean that is conveniently placed behind the player (which will, incidentally, unlock the Seonaidh achievement)—you can get an alternative voice over playing at the start of the game. This alternative voice over is however not necessary for the Seanchaidh achievement.

Walk past the lighthouse and you will have to make your first choice. Will you take the Left Hand Path or the Right Hand Path?
We all know the Left Hand Path is the seemingly easy path, the path of the sinner and the path of sloth, whereas the Right Hand Path leads to true salvation. Hence, we take the Right Hand Path, right?

Following this perilious path you will come to a seemingly dead end where the path has given way to an earth slide (no screenshot, just trust me: you will). You can actually continue your journey by hugging the hill side.

The voice over unlocks after a couple of yards once you approach a cave at your right hand.
(Voice over point 2/10)

You can explore the cave while the voice over plays, but for the next voice over point (henceforth: VOP) continue your path a few yards to find that another land slide has put an abrupt end to your journey. Don't despair however (this is the Right Hand Path after all—only adversity leads to true knowledge), slide down and carry ever onward past the beach.

Eventually, your path will be blocked by a rock formation rising out of the ocean with several boulders alongside it. The third voice over will play once your approach the formation.
(VOP 3/10)

Now the fourth VOP can be easily missed. The game seems to direct you up the cliff once again, this time by the Left Hand Path. However, clearly this is but a trick for the weak of mind.
(do not yet proceed along the Left Hand Path)

Resist the urge to continue past the obvious path, instead inspect the rock formation more closely and surely thou wilt see that a path hath opent up for those who are ever vigilant. Proceed past the rock formation and an image of the golden ratio will appear in the sand.
(image of the golden ratio in the sand)

Once again, press forward past the boulders and the fourth voice over should start to play once you approach the ocean.
(VOP 4/10)

You can explore the crevice to the right, but for out present purpose there is nothing more to glee here. Return from whence thou camest.

Once again approach the Left Hand Path, and this time follow it meandering upwards along the cliff.
(follow the Left Hand Path ye weak of mind)

Once you're a good way up your path will be blocked—again—by a land slide, however, make a hairpin turn and proceed along a wired fence. Halfway along this path the VOP will unlock.
(VOP 5/10)

Wait a bit and enjoy the scenery a bit, because the next VOP is already approaching...

Proceed along the cliff and before long the sixth VOP will unlock.
(VOP 6/10 almost looks like the picture in the last section—yes it's that close)

Meander upward along the cliff side and you will eventually approach the flat atop the hill. Continue until you approach a rock pillar within a field of thistle. The seventh VOP should start to play once you approach it.
(VOP 7/10)

Continue your way and go down the hill again. You will approach a rocky beach with a fishing ship stranded on it. The eighth VOP should unlock once you descend.
(VOP 8/10 notice the cave at the left side)

Now at the eighth VOP, notice how there's a huge—not to be missed—cave at the left side? Can't miss it right? (that's what I said) go there and the penultimate VOP for this chapter will play.
(VOP 9/10)

You can explore the cave, but in all reality you don't need to for this achievement.

Continue past the ditch and through the cliff sides. The tenth—and final for this chapter—VOP will start to play automatically once you proceeded enough along this pathway.
(VOP 10/10)

Press onwards, ever onwards, towards the chapter end.

If you're here also for the Tuir achievement, however, just wait a second because...

You'll be approaching a section with a strange rock pillar formation (actually they form a celtic cross).
This is the location of the first urn. Just explore the right side along the brook and you should see the urn gently lying along some rubble. Click it to return ashes to ashes and dust to dust.
As a special encore: if you approach the left side of the formation, you will be at the top of the crevice where the fourth VOP unlocked. Look down and see how an unfortunate car driver already made the leap of faith... imitate his heroic feat and unlock the Stac an Armin achievement.

Continue your path towards the end... and unlock the Donnelley achievement by doing so.

The Buoy is the second chapter in Dear Esther: Landmark Achievement, its name deriving from the flashing yellow buoy that can be seen floating in the big ocean of nothingness at the beginning of this chapter. There are 11 VOPs here and another (1) urn.

(Because of Steam's strict rules on the maximum amount of characters in each section—effectively baring anyone that surpasses it to edit the section—this chapter is divided into two parts. This first part contains VOP 1-5 which can be found in the beach section of the chapter)

The first VOP is unmissable, starting as soon as you begin this chapter.

Now before we go to the second VOP, we have a difficult decision to make: Left Hand or Right Hand Path? Notice how the choice has been inverted since VOP #5 in the last chapter. Donnely consistently chooses the Left Hand Path over the Right Hand Path: accentuating his decline into madness.
(Donnely follows the Left Hand Path)

Follow the path at the left side and before long you will trigger the second VOP in this chapter.
(VOP 2/11; VOP 3/11 can be seen at the far side near the stranded fishing ship)

Now you can continue the Left Hand Path, but it has a dead end at the top of the cliff overlooking The Buoy.
(The Buoy in yellow's obviously signalling the protagonist's developing sickness and delirium)

Instead at VOP 2/11 make a sharp turn, because the true path to enlightenment is hidden from view.
(Shot from the Buoy—VOP 2/11 sits upwards just to the right, instead go down the hidden path to the left [right if coming from VOP 2/11])

Now to go to the third VOP, continue the path down and cross over all the way to the other side of the stranded tanker. Do so and, after passing a open red container at the far side of said ship, take a left. There is another dead end here, but the voice over will trigger once you cross the markings in the sand/on the rocks near the end.
(Beach and rock markings triggering VOP 3/11)

From VOP 3/11 backtrack a bit and go past the open red container. Keep following the path while keeping the bluff to your left hand. Once a good 10 yards past the container the third voice over should unlock.
(The open red container as seen from VOP 4/11)

Continue the path forward all the while keeping the slope too your left hand and you will eventually pass between a gulley in the dunes.

(path forward from VOP 4/11)

Here you can choose between following the normal path upwards, or taking the rocky path downwards. Certainly you know by now what to do...
(you follow the rocky path to the right)

Not before long you will see a sinkhole emerge beside the rocky road. The fifth voice over will play once you walk past the sinkhole.
(VOP 5/11)

If you somehow managed not to get Stac an Armin in the first chapter, here is a fine opportunity to do so once more. Just head over to the abyss and look into it and perhaps you can see it also looking into you—you won't lose any progress if you do.

(Because of Steam's strict rules on the maximum amount of characters in each section—effectively baring anyone that surpasses it to edit the section—this chapter is divided into two parts. This second part contains VOP 6-11 which can be found in the hill section of the chapter)

From the infernal pit continue up the bluff on top of which stands a little bothy (lodge). However, make sure to first follow the path towards the right side to the derelict little cottage halfway up the bluff. The voice over should unlock whilst approaching the cottage.
(VOP 6/11)

Follow the path that meanders back and forth gently up the slope towards the bothy. Halfway between the cottage and the lodge the path will veer back towards the right again and the seventh VOP should unlock.
(VOP 7/11)

Now valiantly press onward towards the lodge and enter it. The eighth voice over should unlock as soon as you do.
(broken eggs and ultrasounds on the table... it almost sounds like a riddle)

Exit the bothy at the back (or don't). In any case ignore the run down shed and go down the slope behind the lodge.
(ignore the shed and go down the pathway)

If you missed Stac an Armin earlier, here is (once again) a stellar (albeit less poetic) place to unlock it. Just hurl yourself down the cliff and get it over with. After you're done, continue the descent and halfway down the ninth VOP unlocks.
(VOP 9/11)

And once again we're coming down to the penultimate voice over of the chapter. Well done chap! This one is pretty much unmissable, just follow the defile down the cliff towards the stairs.
(VOP 10/11)

Continue down the steps unless you also want to complete the Tuir achievement...

... in that case steer the left to the hidden path over the boulders. The urn is lying below on the beach. Click it in the sure and certain hope of the resurrection to eternal life.
(poor Jakobson)

From VOP 10/11 continue down the steps and towards the skeleton barge. Enjoy the hidden symbolism here and press on.
(I'm quite sure this rotten mast of the skeleton barge contains an allusion to some hidden imagery)

You will come to a small inlet leading to a cave. The final voice over of this chapter should unlock here.
(VOP 11/11)

For story purposes you can explore the gorge to the right, to see what happened with Donnely's books. Else into the breach! Whilst entering the cave Jakobson should unlock.
(The exit)

So the eponymous chapter The Caves is called so because it features lots of caves. I'm quite sure at the end of chapter 2 and the beginning of chapter 3 we can see some imagery representing birth. A leitmotiv that seems to carry through the chapter... life and death, birth and death; sometimes they're just inches apart.

This duality is further accentuated by the colour scheme in this section, there's lots of contrasting oranges and blues here, alternately symbolizing fire, warmth and life (orange) and water, coldness and death (blue).

The chapter features but 4 hard-to-miss voice overs and 1 urn.

Once again, you can't miss this one. After waking up and coming to his senses this voice over immediately starts to play.
(Orange and blue; classic opposites that invoke such a strong feeling of majesty)

Follow the only way out through the fluorescent teal subterranean passage that leads into another orange/blue cavern.
(An island of hope lost in a sea of sorrow)

Keep to the right while entering the chamber and the second voice over should start playing.
(VOP 2/4)

Walk around the island—and notice how this chamber seems to be a dead end—only to see that a hidden passage next to entryway you came through has opened up.
(Left the entryway; right the marvelously hidden exit)

Up next comes a long atmospheric section where we experience Donnelly's struggle with and journey through hope, sadness and despair. First follow the passage until you across come a big luminescent teal grotto.
Jump down into the sinkhole and be reborn in the pool at the bottom. Crawl out and into the tunnel of despair where even the slightest shimmer of light has been doused in a cold and distant cyan light.
You once again enter the grotto with the sinkhole, now on the opposite side and continue the road towards the faint orange light of hope...
Now at this point you really have to credit the music direction in this game. Certainly, the score in chapter 1 and 2 highlighted certain passages by evoking short senses of mystique, longing, loss and desolation; but here in chapter 3 the really subtle ever present drone just underpins the melancholy and the futility of the protagonists endeavour.

Through the music we finally connect with his bleak and hopeless outlook on life. Just savour the hopelessness of it all until...
... you enter a huge underground cavern that, incidentally, also starts the third voice over (which cannot be missed).

Cross the bridge, and notice how something comes floating down the underground river...
... you can follow it if you want—it'd suggest you do—but there is no achievement associated with it. Just the futility of so many thoughts lost... as tears in the rain.

Now cross the bridge and go left, unless you're here for the Tuir achievement.

If you are, then turn right towards to sandy river bank.
(Gently descend down the sandy slope)

The urn is sitting inconspicuously at the bottom of the slope. Click the urn so that he may shine his light upon thee and you've collected 3 out of 4 urns for the achievement.
(Urn 3/4)

As you turn left from the bridge you will slowly start to make your way upwards. Continue along the path until you can enter a spelunk to your right. Do so and turn immediately right. You'll enter a chamber with a puddle and the final (and unmissable) voice over of this chapter will roll.
Make sure NOT to walk through the puddle—which happens to be a pool—during the voice over or it might not register (happened to me during the writing of this walkthrough). If you do, don't despair: as long as you have 24 voice overs registered at this point you should be fine.

Exit through the orange tunnel at the back and start your ascend into madness.
(Surely, whoever had time to scribble this all down was a either a madman or a genius—or both)

Keep following the path, take a dive here, jump through a hoop there, et cetera, and eventually at the end the Esther achievement will unlock.
(And we're back where we started... at a giant tunnel with light at the end—alright perhaps there was no light at the beginning only darkness...)

At last we enter the final chapter of Dear Esther:Landmark Edition, which is aptly references as The Beacon. I let you figure out for yourself why it's called that way. This section contains another 11 voice overs and 1 urn. For a total of 36 voice overs at the end of the game (34 are needed for the Seanchaidh achievement) and a total of 4 urns (to complete the Tuir achievement).

Now, unlike in the last chapter where you really have to try to miss any of the voice overs, in this chapter a few of the early ones are missable. Hence, I'll keep my banter to a minimum and let you explore and experience the chapter on your own.

However, once again credit has to be given to the musical score that just fits the scenery and the narrative perfectly. Providing a backdrop crescendo that evokes both feelings of hope and despair—one would almost say: purpose.

In stark contrast to the last chapters, this one does not start with a voice over. Instead, make sure to keep to the right side of the cave you start in and the first voice over should unlock as soon as you step outside.
(VOP 1/11)

Notice how VOP 1/11 is located on a path along the ridge? Well the second voice over isn't, so unless you feel like exploring, jump down and continue the path below the ridge. Pass through the stone arch unto the beach and follow the conveniently placed candles. The second VOP should unlock shortly. Keep following the shore unless you want to complete Tuir...
(VOP 2/11)

For the fourth and final urn, just look to your right from the second VOP and you should see a bunch of wooden planking across the beach...
... and the fourth urn.
Click it so that the content will all be worn down into granules, washed into the sea and dispersed and congratulations on completing the Tuir achievement!

Also, if you did not finish the Seonaidh achievement earlier... just do yourself a favour and head out for a nice refreshing swim in the ocean.

Continue past several boulders and head down the beach whilst following the trail of single candles; get distracted (or don't) by the cliff side shrines and press on until you have to pass a small gullet between the cliff and a rock formation on the beach, where the third VOP should start to play.
(VOP 3/11)

Keep following the cliff side trail, there is not much else you can do, and the fourth VOP will unlock.
(VOP 4/11)

You can explore here, but eventually you will have to follow the trail towards The Beacon. Keep following the cliffside trail and while coming across a small crest the fifth VOP should unlock.
(VOP 5/11)

Donnelly has a meeting with destiny, thus not much we can do except enjoying the scenery and pressing valiantly forward. Eventually you'll pass a gullet into a recline at the end of which a candle is burning before a right turn. The sixth voice over should start here.
(VOP 6/11)

You can't really miss this one. Just keep following the trail towards the beacon and the seventh seal should break once you pass a narrow gorge.
(VOP 7/11)

After the narrow gullet you will come at a small junction with a bunker at your right hand side.
(Sir, is that a bunker in your pocket or...)

Follow the path towards the bunker a couple of yards and the eighth voice over will start.
(VOP 8/11)

Can't miss this one either. Just proceed towards the top and on the crumpled stairs it should unlock.
(VOP 9/11)

If you followed this guide and all the voice overs registered than the Seanchaidh achievement should unlock here. However, if it doesn't don't despair, there is two more VOPs.

Continue towards the top, climb the broad concrete stairs and follow the chain-link fence until you're just a couple of yards under the beacon platform. The penultimate voice over of the game should unlock once you approach the concrete stairs to the platform.
(VOP 10/11)

Because one of the voice overs failed to register for me, this is actually where I unlocked the Seanchaidh achievement in my playthrough.

Well this is the point where all our faiths intersect; there's only one thing left to do. Congratulations on playing through Dear Esther and enjoy the show. The Paul achievement should unlock at the same time.
(VOP 11/11)

Well that's it. I certainly hope this guide was as helpful to you as writing it was for me. Thanks for reading.

Addendum January 15, 2020: Shout out to everybody still playing this in 2020. I know you guys couldn't open the screenshots before (I had them set on friends only...) so I decided to remedy that situation for the New Year. So now you can finally enjoy your full Seanchaidh + Tuir achievement in stunning 1920x1080 FullHD.