It is a gradual system. First you let them get used to seeing the harness by laying it near their food bowl. Let them sniff it and even play with it at first for a few days. You want to be sure you get the right size and that it won't slip off, because if you don't check it you might lose them outside and that would be horrible.
Once they get used to seeing the harness, you now need to let them see the lead- doing the same thing, leaving it laying around the house on their bed, near their food bowl.
Then you slip it on but don't fasten it- they will wiggle right out, let them, then let them go, do this once a day for about a week until they just could care less about the harness.
Once they accept it, then you fasten it- not tight, but snug, let them run around the house (supervised) with it on. They will roll and run and try to shake it off. Leave it on for 5 minutes, take it off, repeat again every day for a week-
Now time for the lead, put the harness on, snap on the lead and follow them around the house, let them go where they want to go, and you just keep up. Leave it on for 5 minutes, then remove all the gear, do this several times a day for a week.
The first time you go outside (have treats in your pockets) Follow the cat- don't pull or tug, let the cat explore or lay down or do whatever, but no interference from you in any way, you just hang on and follow along. Do this once a day for a week- then go on your outing. Make sure that you follow the cat right at first, unless she is headed for danger- once you get a few feet from you home, reach down and scoop kitty up, praise and pet her and walk home put her inside take off her gear.