Maintaining your patience, and strict avoidance of punishment or coercion, is paramount to success and happiness for all involved. Kittens adapt more quickly to the walking concept than do older cats that are in complete control of their lives. BUT our feline masters can learn to associate the harness and leash with the pleasure of outdoor excursions. Yes, you must use a feline harness with an upper ring for snapping on the leash. Select as simple and lightweight a harness as possible, but use one.
Cats are adept at slipping backwards out of a snug collar when it is attached to a leash.
Believe me, I speak from experience. Every tiny step of progress towards the walking goal should be rewarded with praise and perhaps a food treat. Never, ever punish or scold your cat when they are resisting the leash. Cats do not respond well to negative reinforcement. Your mutual learning experience begins indoors.