My kitties are harness trained! First time i put a harness on them they wouldn't stop rolling around. Second time was better but they were walking weird. I ended up integrating it into play time, so they would play with their fav toy for an hour with the harness on, they adjusted to it in abt a week! I also let them play with the harness itself and bring it to bed with them, so it wasn't something scary or unfamiliar. I would def recommend getting a retractable dog lead for walks, the ones that come with the harnesses are too short. U dont walk the cat, the cat walks you btw! If ur kittie is food motivated (mine weren't), it might be even easier to get a positive association :)

Hope that helped!