first you measure the distance from centre to center of a twist, on the good rail, say its ( 2") ,you mark out your new rail in 2" sections all its length, then you make a small, "nick" on one corner (to take sewing thread)and another at the opposte end and corner, now fix the thread to one end,and twist it around the stock so it crosses your 2" markes,till at the other end,and fix it in the pre, made "nick" (you made there),now with a pencil, mark it (by following the thread)rotate till at the opposite end. now this pencil line is made into a saw cut( follow your good rail for the depth) then a rasp can be used to follow this saw cut. etc,etc.