Ingredients:

• 1 blunt instrument of any kind
  
• 1 or more heads
  
• optional glowsticks for decoration and afterparty rave

TURN OFF YOUR FLASHLIGHT before entering any unknown areas or opening doors. When it comes to light, they all behave like the Witches from Left 4 Dead. Whenever you reach a door, remind teammates: "lights and hammers!" It's always fun when someone forgets to put away their gun before charging their hammer.

Make sure to equip your melee instrument. Tapping the melee key while your melee weapon is stowed will rapidly equip it.

NB: Further melee key presses will perform the swing, but it will be a weak swing and you don't wanna do that.

You can safely move when they are dark and silent. However, if they're making croaking sounds, or softly glow with a pale light, they are more perceptive of their surroundings and if you're still moving when you're near to them, they will begin rapidly and noisily pulsating. If all players stop moving entirely, they will calm down and you can continue when they go dark again.

You make a lot less movement noise when crouched/crawling.

NB: If they turn red, better finish them QUICKLY before they raise the alarm.

If there are are no other monsters within a 2-meter/2-yard radius of the target, you can solo-kill the target.

If there are multiple targets within that close proximity, then you'll need to coordinate with teammates to perform synchronized takedowns. (there's a section on that, later.)

Use Glow Sticks to safely illuminate the area around a target to ensure there are no obstructions to your movement and that there's no additional monsters that you are unaware of.

If you must use your flashlight, then use it in short pulses / bursts / flashes for obvious reasons.

Advance on the target, stopping only when it begins its rapid pulsation. Walking makes less noise than running, and crouching makes less noise than walking.

When you're within striking distance, your circular reticle will become smaller, and you're ready for The Execution.

1: Hit them in the right spot:


Diagram via Gamepedia GTFO Wiki[gtfo.gamepedia.com]

Hits from behind will always score additional damage, especially if the blow lands on the head.
Optimal blows to normal sleepers often require only half-charged hits, while sub-optimal blows to the chest may require a full-charged blow for a one-hit-kill. (via FestiveRocket)

Hold down LMB to charge the swing until the gauge is full, release to swing. I always hold it down for a little bit longer just to be sure. If you hold it for a very long time, you will auto-swing. You may want to practice this a couple times just to see what it's like.

2: BE PATIENT!
Sometimes the Sleepers move around in their sleep, which makes this tricky. Be patient and wait for them to settle down before committing. Also, you may have to walk around them if you're behind them in order to get a clear shot. You only get one shot, so make sure it's a good one.

Every now and then, someone neglects to do this, goes for a side shot and the enemy survives the hit. It could be that the hammer swing uses an arc to detect the hits and that throws things off. One user has reported that aiming off (roughly 100 degrees) to the side helps with this. Further experimentation required to make this conclusive.

You didn't fully charge the swing and now the enemy is glowing red and making noises!

or:

The monster moved its head out of the way while you were swinging and now the enemy is glowing red and making noises!

Quickly hit it again and again until you're sure it's dead or your cover has been blown. Right-clicking with melee performs a shove and can stagger smaller enemies, buying you some time to finish up with another swing. If you can kill it before it screams, you might still be able to avert disaster. Look around to see if other enemies are turning red, and if it's just one other, repeat your shove/swing process.

or:

A nearby monster was alerted!

Slam it over and over until it's dead. Don't give up on stealth so easily - If the other enemies haven't turned red, you still have a chance to keep the cover of stealth. Coordinating with teammates to perform coordinated takedowns is very helpful in that respect.

or:

You were approaching MULTIPLE monsters and any one of them did its "pulsating" thing and you were still moving.

It's easy to lose your cool on The Approach. The more monsters you're approaching at once, the more patient you have to be. Everyone on the team has to be patient when you're clearing rooms. As long as the cover of stealth hasn't been broken, time is on your side.

or:

The game glitches and a distant enemy is aggro'd for no particular reason after killing a different one.

Prepare to go loud. Call it out to your teammates to sprint out of there and don't forget to close the door when the last man's out. If someone's holding a tracker, tag all the enemies before the door busts, and turn on your flashlights.

Sometimes you don't have the luxury of dealing with an isolated target. Sometimes there are two, three, or even more sleepers standing in close proximity to one another.

So, you and your teammates will *definitely* need to be in the same voice chatroom for this to work:

This is the script that worked pretty well for our team:

• "Is everyone in position?" -- you have to be patient.
  
• "We will begin CHARGING at the end of the countdown. Synchronize on MY countdown..."
  
• "Three, two, one, CHARGE." -- Do not release your fire button. Just let the hammer auto-swing.

Scouts make The Approach a little bit more tricky:

That's the only extra rule you have to follow is:

Rule #1: DON'T TOUCHA THA SPAGHET.

First, observe the Scout at a distance. Wait for the scout to retract its tendrils. Crouch-walk up to it and charge up a little early on the way there, and then just hit the scout while it is in motion. If it stops moving, you'd better nail it on the head before the spaghetti comes out or it'll summon a horde.

That's all there is to killing a walking spaghetti monster.

Apparently you can also C-foam them (full charge, 5-6 small blobs, or a grenade) to freeze them for a hammer/head shot.

And, apparently MINES are capable of killing them silently despite exploding loudly. They will probably fix this later. They're like the Hush-A-Boom from Rocky & Bullwinkle.

Hammer hits stagger them briefly, but you better finish them before they do their glowy charge-scream.

If they're charging up (glowing blue/green) they're invulnerable. Don't waste your ammo.

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BREAKING via @ForgottenB:
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• Whenever you are in close proximity to the scout while it's tendrils are out, it will retract and come back out nearly instantaneously after retracting.
  
• It's also worth noting that sometimes the angle at which you hit the scout plays a big part in making sure you don't miss

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BREAKING via @BeatenByAGirl :3:
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• You might be able to get close enough to the scout and stay crouched to avoid getting detected by its tendrils.

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BREAKING via @Ze Ubermensch
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• If a tendril touches you, you might be able to melee-bop it to death before it screams.
  
• The knife can one-shot a scout but only if it's a fully charged hit to the back of the head.
  
• Scouts seem to have different rules for being alerted in that they don't have the pulsation state that detects movement. Gunshots and insufficient damage to kill the monster will still set it off.
  
• Flashlights and loud walking sounds will cause it to release its tendrils, which might be used to your advantage, since it alters the timing of its tendril-search.

Get four people together and coordinate a charge hit on him. You can also C-Foam them with a full charge or a C-Foam grenade for a follow-up with 4 hammer hits.

NB: Some people report that his swollen belly is one of his weakpoints. Having different team members target his belly can help rather than having everyone target his head. My personal theory is that if all four members go for a headshot, it's likely that the first couple of hits may destroy his head, causing the other hammer strikes to miss entirely.

Spikers

Fullly-charged hammer swing to the back while standing directly behind the target, instead of the head to avoid taking toxic damage. Don't try to hit their sides. It gets tricky if their back is obstructed.

The C-Foam launcher is 10,000% stealthy!

2 small blobs will freeze any normal enemy for about 7 seconds and render it completely unaware to anything going on around it. Getting hit by the foam does not aggro it, and when it unfoams, it will remain dormant.

You can foam an enemy, kill a target right next to it, fire your weapon into the air right next to it (as long as there are no other unfoamed targets), and when the enemy unfoams, it won't give a rat's ass.

Very useful if there are multiple targets next to each other and you are short a person for the synchronized takedown.

OK, let's say you have three enemies right next to each other, and unfortunately, they're all taking turns doing their annoying breathing and thumping in a staggered fashion and it's making sneaking very difficult.

You can blink your flashlight briefly on an enemy to cause it to prematurely enter its "thumping" cycle. You can use this on multiple targets to force them to 'synchronize', which makes sneaking a lot easier. Careful you don't blink an enemy that is thumping, though. You could cause it to awaken.

NB: This bug will be patched at the next major update!

A couple of users have reported that at the time of this writing, MINES are SILENT, and are effective for taking down scouts in a stealthy fashion!

I have no doubt that this will be patched or fixed in a later update so get your kicks in while you can.