The best thing to do to get your little one used to a harness is while you are with her inside the home, put the harness on let her wear it throughout the day, after a day or so, add the leash, have her fetch a ball, with the leash on, take her naps with it, eat with it, keeping an eye on her to make sure she does not get tangled on something.... pretty soon they forget the leash and harness are even on. When you walk her with the leash, take tiny treats with you and say, good girl Walk. Give a treat. Time, love, treats and patience.
Best to you.