I decided to create this guide because a friend wanted a guide for how to reach all of the game's endings, but every other guide I could find also included information about how to reach 100% completion of CGs, TIPS, and achievements, which he didn't care about and cluttered up the guide.

Play the game as you wish, and answer any text messages however you want.

When you receive a text message from Kurisu with the subject "Did you write it?", create a new save file before responding to it, and lock the save file so you don't overwrite it later.
After doing this, DO NOT respond to the message with "scientist". You can ignore this text or send any other response, just not "scientist".

Play the game as you wish, and answer any text messages however you want.

When you are prompted to send a D-Mail, save your game and do not overwrite it.

If you choose not to send the D-Mail, you will reach Suzuha's ending, after which you can load back to the save you made and choose to send the D-Mail, which will let you continue the game.

When you are prompted to send a D-Mail, save your game and do not overwrite it.

If you choose not to send the D-Mail, you will reach Faris's ending, after which you can load back to the save you made and choose to send the D-Mail, which will let you continue the game.

When you are prompted to call the PhoneWave, save your game and do not overwrite it.

If you choose not to call it, you will reach Luka's ending, after which you can load back to the save you made and choose to call the PhoneWave, which will let you continue the game.

Play the game as you wish, and answer any text messages however you want.

However, when you are prompted to send a D-Mail, if you choose not to send it, you will see an extra scene that is definitely worth watching.
You don't miss anything by not sending it, so there is no reason to save before doing so.

You may make any choices you want, and at the end of this chapter, you will reach Mayuri's ending.

After this ending, load back to the save you made in chapter 4 to begin your second playthrough. In this second playthrough, you can and should make use of the game's skip feature, which will by default pause when a text message is recieved.

This time around, when you receive the text message from Kurisu titled "Did you write it?", respond with "scientist". Then in subsequent messages with Kurisu, make the following choices:

TITLE -> RESPONSE
"Scientist??" -> "bad scientist"
"Bad Scientist" -> "do better"
"Home Alone" -> "outsider"
"Ehhh?" -> "disgrace"
"Staring at my hand" -> "why am I still here?"

When you receive and open the message from Kurisu titled "Being here", you will unlock the first true ending flag. There is no indicator of this, but if you responded correctly, then it worked.

When you receive the message from Kurisu titled "Plans", respond with "traveling expenses". Then simply open her two subsequent messages "Idiot!" and "[Emoticon]", which require no response, and you will receive the second true ending flag.

If you do not receive the message titled "Plans", then it means that you did not receive the previous true ending flag.

Make any choices you like in this chapter, but when you are prompted to send a D-Mail, choose to send it in order to continue on with the game.

When you receive the message from Kurisu titled "The key [Emoticon]", reply with "I don't have a key", and then respond to her subsequent messages as such:

TITLE -> RESPONSE
"[Emoticon]" -> "security"
"You're delusional, I know" -> "reality"

Then, after opening Kurisu's text titled "Reality", you will receive the third true ending flag.

When you are prompted to send a D-Mail, choose to send it in order to continue on with the game.

When you receive Kurisu's message titled "Explain yourself!", no response is required to progress.
When you open her message titled "From an unpopular girl", you will receive the fourth true ending flag.

When you are prompted to call the PhoneWave, choose to call it in order to continue on with the story.

When you receive Kurisu's message titled "Contact me, idiot!", reply with "worried". Then, when you open her message titled "Re: Re: Contact me, idiot!", you will receive the fifth true ending flag.

When you receive the message from Kurisu titled "Where are you now?", save the game to a new slot and lock it to ensure you don't overwrite it. Then, DO NOT reply to the message with "where you are".
After doing this, continue through the chapter and you will reach Kurisu's ending.

Then, load back to the save you made, and reply to Kurisu's message with "where you are", which will get you the sixth and final true ending flag.
After doing this, if you continue through the chapter, you will reach the game's True Ending.