all routes start when you have to choose who to stay with for the night during the stargazing (and for torulf you also have to say that you are comfortable around him when he flirts with you). you have to talk to them a few times throughout day 1 to be able to pick them. this is with the exception of gudrun and klaus, who are "special" routes that you can probably find the steps somewhere in the comments.
also note that jorgen's route splits into 2 paths on day 4, and torulf's path splits into 3 (4?) different routes (dont know when sorry i havent played his route).