Building affection over the course of the game is fairly straightforward. The choices are marked in red or orange with a "+Affection" marker. Just choose enough of the respective dialogue options to unlock the character's offering you their romance option. You can still decline.

Phoenix ending
Be nice to Phoenix. Show them the work, invite them to the café, ask follow up questions, listen to them when they open up to you, lend them the manga...

Joyce ending
There are several options where you can gain affection with Joyce by staying home and validating them as a person.

This ending does require you to get the following achievement by successfully rescuing Joyce from Bell Tech.
The right answers are:

• Are you afraid of running out of time?
  
• An app
  
• Our unique skillsets
  
• Use fake names
  
• Make something up
  
  
• I'm here to talk
  
• You're just as important as anyone else
  
• Because I want to understand
  
• You could have talked to me

Teal ending
Simply decline every other romance option if it's being offered.

Get the job at Naughty Cat. You need to work on your artwork at least three times to win the first round and then give the right answers in the second round:

• Photo-bashing
  
• Yellow
  
• Ask what is wrong with them/Remove the details(?)
  
• Most communication is digital these days
  
• They actually don't have a gender
  
• They make the AI appear more mechanical
  
• Did you struggle with the time limit? Yes
  
• You're one of the best
  
• My current skill level

By this point you have the job pretty much in the bag. It doesn't matter if you prepare for the second round at all.

Pick up extra shifts (at least 4) at the book shop and fail the competition to be made manager at the book shop. How well you deal with the customers doesn't matter at all.

You're quite a bit short on rent when the game starts. Not working on your artwork for the competition and declining shifts in the book shop on your off days will do the trick.

Play through your guild's story by choosing to login and game four times. Remember to pick a username and password you won't forget.

Agree every time Ms. Cardhover asks you to come in to work on your days off.

Fail to rescue Joyce from Bell Tech. See the right answers at "To the Rescue" and simply choose the wrong ones. Or just outright decline when Zen offers to go rescue them.

This achievement unlocks after you got every other achievement in the game.