top left to bottom right on the size that you are trimming the piece to. in this sample, it would be 4″.
rotate the piece and line up the cut edge exactly on the 4″ line of the ruler.
first, line up the diagonal line that goes from the top left to the bottom right on the size that you want to trim to. for example, 4 1/2″. also, line up the box that the square fits in which is half the size that you are trimming to, in this case 2 1/4″ trim.
rotate, line up the piece on 4 1/2″ and trim.
this is where the companion ruler comes in. you need 