Take a look here. Rafalfett has a good template and some images. You will basically have to cut out the shape (using the template) and sculpt the dent back in. (I know I'm over simplifying it.)
RafalFett definitely lays it out well. I added my dent by covering the hole with a large strip of masking tape. How i did it was. Cut the hole. Loosely lay the masking tape over the hole using a finger i pushed it down the desired depth. Secured the tape along the outside of the dent, and used filler inside the helmet laid on top of the tape.