I never really broke Moo in on a ferret harness. I just put it on him and went. He's a very laid back and mellow cat. LOVES people and being the center of attention.

I put Maya Linn in a harness and walked her around my livingroom first. Then we went outside. It didn't take long.

I would suggest the latter method. Get them used to the harness first. Then the rest is a cake walk