What Bluie said is good but, to add on:
You should hold the treat in front of her nose, and touch her back where the harness would be. Let her have the treat. Leave her alone for a few minutes. Come back, and simply put the harness on the floor in front of you. Call her over to you, and put the treat on the harness. Let her eat it and walk away. Wait another few minutes. Call her back, and hold the harness in one hand and a tasty treat in the other. Hold the treat firmly while she is nibbling on it. Place the harness on her back at the same time, move slowly. Gently slide her legs through, while she's still distracted.
Depending on how scared she is, this may take a couple attempts. If she becomes uninterested in the Milk Bones or whatever, use blocks of cheese or even some lunchmeat.