To start the cat needs to just get used to wearing the harness!! Put it on her for small periods of time and just let her walk around the house in it. Then start leaving it on for longer! When she starts walking normally with the harness and doesn't seem effected (or affected?? lol) by wearing it anymore you can start attaching the leash to it while she is around the house. Eventually start following her with the leash on, then grab the leash while following her but do not cause any resistance. Once she is feeling comfortable you can keep progressing until she is completely comfortable with all that and can handle some resistance! After that you can start going outside.
Roxy LOVES going out on walks!!!!