Ok, after playing through this game at least 4 times...I think I have it together enough to help everyone else out 100% this nightmaregame. I did spoiler out images and certain things to give you the most spoiler free walkthrough I could write for you.

***I did use a chart from Gamefaqs to help with poetry writing...other than that, everything else is from yours truly.***

Description of the Game via Steam:
Hi, Monika here!
Welcome to the Literature Club! It's always been a dream of mine to make something special out of the things I love. Now that you're a club member, you can help me make that dream come true in this cute game!

Every day is full of chit-chat and fun activities with all of my adorable and unique club members:

Sayori, the youthful bundle of sunshine who values happiness the most;
Natsuki, the deceivingly cute girl who packs an assertive punch;
Yuri, the timid and mysterious one who finds comfort in the world of books;
...And, of course, Monika, the leader of the club! That's me!



I'm super excited for you to make friends with everyone and help the Literature Club become a more intimate place for all my members. But I can tell already that you're a sweetheart—will you promise to spend the most time with me? ♥

This game is not suitable for children
or those who are easily disturbed.
Just Monika

Poem Word Mechanics:

Three times per each route you will have to write out a "poem" using 20 words. When you choose the right word for whichever girl you are trying to impress, she will jump up in happiness. You'll need to do it successfully a total of 9 times throughout the game (3 for each girl).

**During your second round playthrough, regardless if it's during the Bad or Good ending quest--- during the third poem, if you start choosing words that Sayori would love, you will get a glitched word on the paper...if you click on it, you will get a glitched poem writing session. If you click on Sayori specific words after the glitch you will get noises and voices.
**


Below is a table (from GameFaqs-Eevee-Trainer) that give you a pretty good idea of which word to pick for whichever of the three girls:
Words: -Sayori -Natsuki -Yuri
Adventure + 3 (♥) + 2 + 1
Afterimage + 1 + 1 + 3 (♥)
Agonizing + 2 + 1 + 3 (♥)
Alone + 3 (♥) + 1 + 2
Amazing + 3 (♥) + 2 + 1
Ambient + 2 + 1 + 3 (♥)
Analysis + 1 + 1 + 3 (♥)
Anger + 2 + 3 (♥) + 1
Anime + 2 + 3 (♥) + 1
Anxiety + 2 + 1 + 3 (♥)
Atone + 2 + 1 + 3 (♥)
Aura + 2 + 1 + 3 (♥)
Awesome + 3 (♥) + 2 + 1
Beauty + 3 (♥) + 2 + 1
Bed + 3 (♥) + 2 + 1
Blanket + 1 + 3 (♥) + 2
Bliss + 3 (♥) + 2 + 1
Boop + 2 + 3 (♥) + 1
Bouncy + 2 + 3 (♥) + 1
Breathe + 1 + 2 + 3 (♥)
Broken + 3 (♥) + 1 + 2
Bubbles + 2 + 3 (♥) + 1
Bunny + 2 + 3 (♥) + 1
Cage + 1 + 2 + 3 (♥)
Calm + 3 (♥) + 1 + 2
Candy + 2 + 3 (♥) + 1
Captive + 2 + 1 + 3 (♥)
Charm + 3 (♥) + 2 + 1
Cheeks + 2 + 3 (♥) + 1
Cheer + 3 (♥) + 2 + 1
Childhood + 3 (♥) + 2 + 1
Chocolate + 2 + 3 (♥) + 1
Climax + 1 + 2 + 3 (♥)
Clouds + 2 + 3 (♥) + 1
Clumsy + 3 (♥) + 2 + 1
Color + 3 (♥) + 2 + 1
Comfort + 3 (♥) + 2 + 1
Contamination + 1 + 1 + 3 (♥)
Covet + 1 + 1 + 3 (♥)
Crimson + 1 + 1 + 3 (♥)
Cry + 3 (♥) + 1 + 2
Cute + 2 + 3 (♥) + 1
Dance + 3 (♥) + 2 + 1
Dark + 3 (♥) + 1 + 2
Daydream + 3 (♥) + 1 + 2
Dazzle + 3 (♥) + 2 + 1
Death + 3 (♥) + 1 + 2
Defeat + 3 (♥) + 1 + 2
Depression + 3 (♥) + 1 + 2
Desire + 1 + 2 + 3 (♥)
Despise + 2 + 1 + 3 (♥)
Destiny + 2 + 1 + 3 (♥)
Determination + 1 + 1 + 3 (♥)
Disarray + 1 + 1 + 3 (♥)
Disaster + 2 + 1 + 3 (♥)
Disoriented + 1 + 1 + 3 (♥)
Disown + 1 + 1 + 3 (♥)
Doki-Doki + 2 + 3 (♥) + 1
Dream + 2 + 2 + 3 (♥)
Effulgent + 1 + 1 + 3 (♥)
Electricity + 1 + 2 + 3 (♥)
Email + 2 + 3 (♥) + 1
Embrace + 3 (♥) + 1 + 2
Empty + 3 (♥) + 1 + 2
Entropy + 1 + 1 + 3 (♥)
Essence + 2 + 1 + 3 (♥)
Eternity + 2 + 1 + 3 (♥)
Excitement + 3 (♥) + 2 + 1
Existence + 2 + 1 + 3 (♥)
Explode + 1 + 2 + 3 (♥)
Extraordinary + 3 (♥) + 2 + 1
Extreme + 1 + 1 + 3 (♥)
Family + 3 (♥) + 2 + 1
Fantasy + 1 + 3 (♥) + 2
Fear + 3 (♥) + 1 + 2
Feather + 3 (♥) + 2 + 1
Fester + 2 + 1 + 3 (♥)
Fickle + 1 + 1 + 3 (♥)
Fireflies + 3 (♥) + 2 + 1
Fireworks + 3 (♥) + 2 + 1
Flee + 2 + 1 + 3 (♥)
Flower + 3 (♥) + 2 + 1
Fluffy + 2 + 3 (♥) + 1
Flying + 3 (♥) + 2 + 1
Forgive + 3 (♥) + 1 + 2
Friends + 3 (♥) + 2 + 1
Frightening + 2 + 1 + 3 (♥)
Fun + 3 (♥) + 2 + 1
Games + 2 + 3 (♥) + 1
Giggle + 2 + 3 (♥) + 1
Graveyard + 2 + 1 + 3 (♥)
Grief + 3 (♥) + 1 + 2
Hair + 2 + 3 (♥) + 1
Happiness + 3 (♥) + 2 + 1
Headphones + 2 + 3 (♥) + 1
Heartbeat + 1 + 3 (♥) + 2
Heart + 3 (♥) + 2 + 1
Heavensent + 2 + 1 + 3 (♥)
Holiday + 3 (♥) + 2 + 1
Hopeless + 3 (♥) + 1 + 2
Hope + 3 (♥) + 1 + 2
Hop + 2 + 3 (♥) + 1
Horror + 2 + 1 + 3 (♥)
Hurt + 3 (♥) + 1 + 2
Imagination + 2 + 1 + 3 (♥)
Incapable + 2 + 1 + 3 (♥)
Incongruent + 1 + 1 + 3 (♥)
Infallible + 1 + 1 + 3 (♥)
Inferno + 2 + 1 + 3 (♥)
Infinite + 2 + 1 + 3 (♥)
Insight + 2 + 1 + 3 (♥)
Intellectual + 1 + 1 + 3 (♥)
Journey + 2 + 1 + 3 (♥)
Joy + 3 (♥) + 1 + 2
Judgment + 1 + 1 + 3 (♥)
Jumpy + 2 + 3 (♥) + 1
Jump + 2 + 3 (♥) + 1
Kawaii + 2 + 3 (♥) + 1
Kiss + 1 + 3 (♥) + 2
Kitty + 2 + 3 (♥) + 1
Landscape + 2 + 1 + 3 (♥)
Laugh + 3 (♥) + 2 + 1
Lazy + 3 (♥) + 2 + 1
Lipstick + 1 + 3 (♥) + 2
Lollipop + 2 + 3 (♥) + 1
Loud + 3 (♥) + 2 + 1
Love + 3 (♥) + 2 + 1
Lucky + 3 (♥) + 2 + 1
Lust + 1 + 2 + 3 (♥)
Marriage + 3 (♥) + 2 + 1
Marshmallow + 2 + 3 (♥) + 1
Massacre + 2 + 1 + 3 (♥)
Meager + 1 + 1 + 3 (♥)
Melancholy + 2 + 1 + 3 (♥)
Melody + 2 + 3 (♥) + 1
Memories + 3 (♥) + 2 + 1
Milk + 2 + 3 (♥) + 1
Misery + 3 (♥) + 1 + 2
Misfortune + 3 (♥) + 1 + 2
Mouse + 1 + 3 (♥) + 2
Music + 3 (♥) + 2 + 1
Nature + 3 (♥) + 2 + 1
Nibble + 2 + 3 (♥) + 1
Nightgown + 1 + 3 (♥) + 2
Ocean + 3 (♥) + 2 + 1
Pain + 3 (♥) + 1 + 2
Papa + 2 + 3 (♥) + 1
Parfait + 2 + 3 (♥) + 1
Party + 3 (♥) + 2 + 1
Passion + 3 (♥) + 2 + 1
Peaceful + 3 (♥) + 1 + 2
Peace + 2 + 3 (♥) + 1
Philosophy + 1 + 1 + 3 (♥)
Pink + 2 + 3 (♥) + 1
Playground + 2 + 3 (♥) + 1
Play + 3 (♥) + 2 + 1
Pleasure + 1 + 2 + 3 (♥)
Poof + 2 + 3 (♥) + 1
Portrait + 2 + 1 + 3 (♥)
Pout + 2 + 3 (♥) + 1
Prayer + 3 (♥) + 1 + 2
Precious + 3 (♥) + 2 + 1
Promise + 3 (♥) + 2 + 1
Puppy + 2 + 3 (♥) + 1
Pure + 1 + 3 (♥) + 2
Question + 1 + 2 + 3 (♥)
Rainbow + 3 (♥) + 2 + 1
Raincloud + 3 (♥) + 2 + 1
Raindrops + 2 + 1 + 3 (♥)
Ribbon + 2 + 3 (♥) + 1
Romance + 3 (♥) + 2 + 1
Rose + 3 (♥) + 1 + 2
Sadness + 3 (♥) + 2 + 1
Scars + 3 (♥) + 1 + 2
Secretive + 2 + 1 + 3 (♥)
Sensation + 1 + 2 + 3 (♥)
Shame + 3 (♥) + 1 + 2
Shiny + 2 + 3 (♥) + 1
Shopping + 2 + 3 (♥) + 1
Silly + 3 (♥) + 2 + 1
Sing + 3 (♥) + 2 + 1
Skipping + 2 + 3 (♥) + 1
Skirt + 2 + 3 (♥) + 1
Smile + 3 (♥) + 2 + 1
Socks + 2 + 3 (♥) + 1
Sparkle + 3 (♥) + 2 + 1
Special + 3 (♥) + 2 + 1
Spinning + 2 + 3 (♥) + 1
Starscape + 2 + 1 + 3 (♥)
Sticky + 2 + 3 (♥) + 1
Strawberry + 2 + 3 (♥) + 1
Sugar + 2 + 3 (♥) + 1
Suicide + 2 + 1 + 3 (♥)
Summer + 2 + 3 (♥) + 1
Sunny + 3 (♥) + 2 + 1
Sunset + 3 (♥) + 2 + 1
Sweet + 3 (♥) + 2 + 1
Swimsuit + 2 + 3 (♥) + 1
Tears + 3 (♥) + 1 + 2
Tenacious + 1 + 1 + 3 (♥)

Bad Ending:

This is a really easy one to get. You will get this route easily if you do not take advantage of the save/load mechanics. Pretty much just play Sayori's route. Each poem choose words that she would like. After 3 times, you will come to an option to confess to Sayori, make sure you tell her you love her. After a certain scene with Sayori and the school festival, you will see an "END" screen.

"Certain Scene"

Once you start your Second playthrough using the "glitched" New Game option, the story will play out drastically different. You will notice all the girls starting to psychotically break

Here's what your Title Screen will look like...

You will play through until you enter a room with just Monika. You need to listen to her until she suggest just staring into each other's eyes. At that point, go into the DDLC file folder Character, and delete "Monika.chr". There the story will eventually progress to the end. From there, start a new game and play until the credits.

Just Monika

"Also yes protip. Ya don't need to reinstall the game for you to play it again. just go through the files and then go to the "game" file. after that go and delete the "firstrun" folder
Then you'll be all set for another run" -Thank you -[FF]- The Sexelent Spy

Best Ending: Best is subjective honestly

Best Ending:
For the best ending possible, you will have to play through the game 3 times and take advantage of the save/load mechanics. The first playthrough is Sayori, followed by Natsuki and then Yuri. The way to gain their respective paths is by writing poems that each of them would love using certain words and who you choose to hang out with the day before the School Festival.

The main way to get the best ending also, is by gaining 100% of the CG graphics. The game is able to keep track of your collection even if you can't see it.

Playthrough: Sayori
-Save the game at the first poem
-Hang out with Yuri
-Tell Sayori that "I love you". Once you get the CG graphic, load the save file for the first poem. You can go past the scene when the CG ends, but DO NOT GO PAST THAT.

Playthrough: Natsuki
-Reload at the first poem and write one that Natsuki would like.
-Hang out with Natsuki
-After Sayori arrives at your house, and Natsuki leaves---load the first poem file once more.

Playthrough: Yuri
-After loading the first poem for the final time, write a poem that Yuri would like.
-Hang out with Yuri
-Tell Sayori that you love her, and this time play until the "END".

OnlyMonika:
After you get through the three playthroughs you will start a new game. Go through it, just as you would a bad playthrough. But, this time make sure you get the Monika CG before deleting her character file. After this scene right here, delete her character

And after the story "END"s, hit start a new game one last time--and play the good ending.

After the Good Ending ends, you must reinstall the game if you'd like to play another time.
Sayori is best girl, fight me

This is for the ??? or Weird Ending:

For this ending, all you need to do is start a completely new game. Open the DDLC game folder and go to the folder labeled: Character. Delete the "Monika.chr" file. Now, start a "New Game", and you will get the Weird Ending. (It's a very short Sayori scene)

You will not get any additional CGIs for this, so I suggest doing this after the game is completed for you. The only additional information is, if you decide to see this ending you will have to "reinstall" the game in order for the game to be playable.

"Also yes protip. Ya don't need to reinstall the game for you to play it again. just go through the files and then go to the "game" file. after that go and delete the "firstrun" folder
Then you'll be all set for another run" -Thank you -[FF]- The Sexelent Spy

I'm going to "spoiler" this entire section but this is a preview of events/CGs that happen in the game. The game is as nicely illustrated as it is disturbed. Enjoy!
Just Monika...Just Monika...Just Monika..




Just Monika...Just Monika...Just Monika..