Do it really slowly. Start by showing her the headcollar, click and treat, touching her face with it, c&T, then build up to showing her the treat through the headollar, etc, until she has to reach through the headcollar to get a high value treat, until you leave it resting on her nose/face for longer periods of a few seconds, build it up, then finally do the headcollar up. She should learn to associate the headcollar with nice things, playtime whilst wearing it around the house etc. Then do the same with the lead going onto it. Use a light lead - the double ended ones designed for smaller dogs are great as they are strong but lighter so don't drag the head down.