I used this method when making the star on Captain America, hope it works for you on your Batman custom. First paint over the Bat sign you don't want, then find a picture of the Bat sign that you want on a printed Batman picture or a picture you don't need, use an exacto knife to carefully shape it out, then paint it, and stick it well to Batman with the side that you painted, after that carefully remove it with a sculpting tool or any convinent tool, and then fill in the gaps with a small paint brush if necessary, watch not to paint too much on the outside of the Bat sign, but from the inside slowly filling the small gaps on the outside, and at the process you'll be adding additional paint coats. It takes a lot of patience, but you'll like final result. Hope that helps