Not sure I can offer much help. Amazingly, both of these cats just let me put it on and that was it.
I tried with my prior cat and he refused to accept it. He was more hyperactive and neurotic than these guys. I think starting off with a cat who will sit on your lap like a child is the key. Butt on your lap, back legs sticking out front and their back to your stomach. That makes a lot of things easier - putting on the harness, clipping nails, etc. My prior cat would freak in that position. These guys are happy to sit like that.
Then it's just putting it on them and seeing how they react. If they have no reaction, then you're good to go. If they bother at the harness, then start with short practices and distract them with food or toys or tummy rubs while they wear it.
With the leash, the easiest is if they know their name and answer to it (as much as a cat will). Let them lead the walk, but if they are headed towards a bad place, just stop and hold the leash taut. Then call them and give a slight short pull in your direction. Be very relaxed and gentle. And make sure to give them rubs and such while they are on leash, but don't interfere with them exploring safely.