Very carefully!! I managed to get one on my Tonk but she extracted it herself from it within about 3 minutes and ran round like a hooligan expressing her disgust very vocally
I had the great idea of training one of my cats to wear a lovely red harness as I used to take them home to my mum for holidays and Xmas etc. and it would mean they could go outside (theory being they could not run off if in a harness).
That idea lasted about 2 minutes after I walked down the garden with cat on said harness something banged and I had a whirling dervish loudly yowling and screaming on the end of the lead for about the 10 seconds it took for cat to extricate itself from the harness and disappear back down the garden. Harness has never made contact with a cat since
I had my young cat in a harness as a kitten (from 4 months). She was desperate to get out and play with my other cats, so I bought one, put her, in it and took her into the garden wearing it. She hated it at first wondering WTF i'm doing to her, she soon realised her harness meant outdoor play and was a bit less resistant when putting it on. I only did it for a few months until she was neutered, she'd probably kill me if I did that to her now
A friend of mine's cat cannot go out unsupervised, he is not very clever, but when the weather was nice we took him in the garden in his harness to sunbathe. He didn't know what to make of it at first but then he just lay down and slept!
We used to take Henry to the vets in one. The dogs would be very, very confused. He didn't much like it but he prefered it to going in the basket. We originally bought it when he was confined to barracks with an injury but refused to use a litter tray