would be to help your cat get used to a specific harness. Let him sniff it, put it near him and let him get used to its smell. Only then put it on him slowly, making no loud sounds or harsh movements that might scare him. It is very important to make it a positive experience for your feline friend. You don't want your cat to get scared of the harness, as it might make it impossible to use it afterwards. If your kitty is showing uneasiness or anxiety, play with him or distract with treats.
Many cats freeze up and don't want to move if you put something on them, this is why you need to give your cat time to get used wearing harness inside, so it won't stumble, fall or freak out outside, where he can get hurt or ran away from you. It may take from several days to a couple of weeks of training for your cat to feel comfortable enough to move further.
Remember to practice adjusting the harness so it would fit your kitty nice and snug, but not too tight. You must be able to put no more than one finger underneath it. Keep in mind that when frightened, your cat can back out of walking jacket, so it is better to choose the safe ones
, which can't be taken off easily.