To train your cat to walk on a leash first get them used to the harness. Give them positive feelings about wearing the harness (while they wear it: give them treats, play with them, pet them). Get them use to moving with the harness (consider using the clicker technique). Slowly build up their confidence (don’t try to push through too fast).
It takes a bit of patience but cats can learn to walk with a leash and to enjoy doing so.