Hi, been walking lots of cats for lots of years. First get the harnesses that not only go around the neck, but also down around the back of the front legs (little piece connects the neck strap to the tummy piece along the back). He's less likely to strangle or escape that way... dogs have much stronger necks! Then, without the leash itself connected to the harness, just attach the harness so it fits well, which means tight enough so you can just get a finger under both parts, but no more (and it won't catch on anything, and he's less likely to slither out.. they're good at that!). Then just let him wear it for increasing times every day in the house... 10 mins, 20 mins.... til he forgets about it. Once he seems comfortable wearing it, wait for a nice day at a quiet time during the day, sprinkle a little catnip just outside the door but reclose it, bring him to the door and casually (quickly) clip on the leash, open the door and take him out with you. Put him down but don't go anywhere and leave the door open behind you. Being outside is likely to make him forget all about the leash, and he may immed. want to go back in, or else crouch and sniff all around him. Let him be the first one to go anywhere, and follow, don't lead. If he totally freaks out the first time when he's outside, let him go back in before he gets out of the harness. Do it all over again later in the day, and the next, but if after 2-3 tries he's really not interested, be glad you have a house cat! You don't want to end up having him like it so much that he wants to be out all the time and yowls at the door all day, never mind the chance, however small, that something (cars, etc.) scare him enough to get out of the harness and never come back.