The only suggestion I have for you to make sure your triangles are centered on your square...fold each piece in half and crease with your fingernail. Match up your creases and pin as you would for star points. That is, put a pin straight through the center of the crease on the top unit then through the center of the bottom unit perpendicular to your fabric. Hold the pin straight up and pin normally as close as possible to the center pin. Remove center pin and finish pinning your units. This should line up your pieces accuratelty without having to do extra trimming.