I ask the age since what worked with my kitty may not work with an older/wiser cat.
I got the smallest harness & had it when I brought Mandy home at 5wks of age. After a week or so while she adjusted & learned to explore & started to play I put the harness on the floor for her to 'check it out'. Next day we sort of played with it - I dragged it around for her to follow a fe times, than just draped it across her shoulders for a second or two. The next day more of the same & then tried buckling it loosely.
She didn't care for it at first, tried to get it off so I distracted her with some other play. The she let it stay on without fuss.
I left it on here all the time. She is also a completely indoor kitty, 3rd flr apt. so I figured there wasn't anything much she would get tangled up with. Also I was home with her at all times when she was that little. She still didn't much like having it put back on when I did take it off.
That's another story though which I can go into if you would be interested later. First get to the point where she will accept having it put on her.
Good Luck, Betty & MANDY