I use a centerline pin tool I got from tower run down the length of the aileron and trailing edge of the wing to mark the center, then according to the kit instructions I mark the pencil lines top and bottom of the ailerons and trailing edge, in most cases it is appx,1/8 of an inch back, and like Campy said cut your hinge slots first. then all you have to do is use the pencil mark you made and the scribe mark from the tool as a sanding guide. they turn out perfect every time when I do it this way.