Leash/Harness Training Tips?

I'm trying to get my cat used to a harness and leash because I want to bring her to college with me. She's used to being outside already (I've always let her wander since we live in a secluded place). But, she's not a fan of the harness. I need to get her comfortable with it quickly, so I could use some advice. She’ll wear, but just lays down and won’t move until I take it off. I always reward her with a treat when it's on. I'm not sure if it's the harness style that she doesn't like, or if I just need to be patient and keep trying. TIA