Make the leash and harness something associated with FUN an FOOD! Let her get a little bit hungry and give her some YUMMY Treats with the harness in sight perhaps making a very slight show that your getting it and putting it next to you. Small treats eaten fast are the best. Less time eating and more time focused on that next treat and how good they are. Next, put the harness on her but no leash and let her get used to wearing it. Offer her treats and loving and praise her for being so pretty. Before she has had to much of the harness and starts to dislike the situation, remove it nicely and give her more little treats. Repeat the harness process long enough for her to enjoy it, expect it to be a good thing and not mind wearing it. When that is done, bring the leash into the story and again give her treats while having the leash hooked up to her and while she tolerates it. If it goes well after a few times try walking her around the room encourageing her with treats so she enjoys being walked in her new harness and leash. Be sure to take those treats with you when you go outdoors.. and keep in mind this is not a one hour or one day project, this is going to take small steps on your part. If the cat gets stressed after a few minutes, or even looks like it might be getting to that point, stop sooner instead of later. Keep it as good of an experience as possible. Behaviors take a lot of effort to adjust, but it is possible.
These are just My thoughts on what you can try that I feel would work. I have never trained a cat to wear a harness myself.