To get a bad ending you have to play through the game without interacting with ANY of the romanceable characters. Just to be safe, try not to initiate even one of a character's events. Also when speaking to any of them in the spa, try to pick the wrong answer (i.e. when you're in the steam room or whatever it's called with Remi, try to make him uncomfortable and then leave instead of joining him in the shower). Then instead of rescuing you, the characters will allow you to be captured by Hisami. This will trigger Tomoki's bad end. Then after that happens, you can visit each romanceable character llke with the other routes, but this time the endings you will get will not be quite so nice.
To get a perfect ending, you have to maximise a character's affection by giving them gifts right before going out with them. And try to pick the correct responses during the interactions.