Make the experience as pleasant as possible. Associate the leash and harness with play. Drag them around on the floor and let your cat chase them. Put the harness and the leash on your cat while it's inside. The first time you take your cat outside, carry it to a quiet, safe spot in your yard, and gently set it down.
Some cats are more afraid than others.
If you know your cat is startled by loud noises, pick it up when a car drives by. Always pick your cat up if another animal comes into your yard. Gradually, your cat will become used to the sights and sounds in your yard. If it refuses to walk on the leash, have someone else stand in front of it and offer it treats. When your cat does decide to walk, let it lead the way.