Put the harness on your cat, but don’t attach the leash yet. Immediately distract him with tasty treats. Adjust the fit of the harness. You should be able to slip two fingers (but not three or four) between the harness and your cat’s body. Leave the harness on for just a couple minutes, removing it before your cat’s interest in his rewards starts to decrease. Repeat this training daily for several days. If your cat stays relaxed, gradually increase the time the harness is on. If he ever gets upset, distract him with treats and then remove the harness. Try again later with a better reward and take the harness off sooner, before your cat has a chance to get upset.