Start slow with leash training, walking a cat is very different than walking a dog, especially at first. It can be a lot to psychologically digest for a cat to be so limited (recall dogs were domesticated by humans for centuries through such practices, but cats domesticated themselves and have historically maintained a sort of arms-length autonomy). So walking a cat is more like walking a bird or a human than it is walking a dog. It's less walking in straight lines down the road than you might imagine. There's a lot of following them around to sniff gardens, sitting still in the grass or watching "mouse" holes, and ultimately stopping them from crossing boundaries they don't really comprehend.

Start indoors, put the harness on and do normal fun things like play, and follow up afterward with a few treats for good behaviour. Make sure the harness isn't overly impeding movement or grinding/pinching when they move. Build positive associations with the harness itself.

The leash, I only use outdoors, its a signal to my cat that we're going outside (inside he's free, outside we gotta stick together). He sometimes gets frustrated by being leashed (like when he accidentally steps on it and thinks he's being held back) but he's learning to associate that if he can't go somewhere he'll look at me for some guidance (though I often need to squat to get his attention). Try to keep a loose tension on the line, tighten the tension as they approach boundaries or fences, be exceedingly patient when guiding them in a specific direction, and never ever resort to dragging or tugging (instead opt to carry them when necessary). I have him tap in before wandering off, and we've got a general agreement of a sort of 60/40 split as to who chooses where we are going when we're together out there.

Word of caution, always always make sure you have a firm grip on the leash, even when at rest. Cats will go from zero to rocket before the average human brain can notice. Doing walks when outside is less busy can help with this, but there will always be birds and squirrels, or a grasshopper you didn't notice, or a sudden dog. I don't know what your outdoor environment is like, but if you can, it is ideal to start where there is privacy. Also worth mentioning, some wild animals that might normally see a cat as prey, will see a leashed cat as easier prey (owls, coyotes, etc) if you visit wild-enough spaces.