(General) If you aren’t in deep story, it’s not gonna happen. The only way to be in Jumin’s route is to play Deep Story.
(General) If you are going to call anyone back, call back Jumin. Just to make sure you have as many interactions with him as possible.
Praise that cat! Jumin loves his cat. So make sure to praise him as well and agree with him if he starts talking about how great cats are.
Working Jaehee to the bone. If he’s bossing Jaehee around, jump in and agree with him doing so.
Be confident! Make sure that you sound modest and smart. He seems to like this.
Don’t freak out. Jumin is a character who doesn’t know how to express his feelings. So once he starts to do so, he might begin to get very intense. Keep both you and him calm but not snapping at him and staying with him.