I make the piece up, glue , use bone folder and tacks if need be and when it's dry I'll trim it.I personally use a round knife to trim, but whatever works for you- razor blade, scalpel, etc. For straight raised strap work I add 1/4" to the top piece- but don't hold me to that, it depends on what your doing and I haven't been at this that long!. Pics of 1/2" raised straight strap and a shaped martingale drop. I'm sure those with more experience will chime in.