I think you are asking basically about the sandwich method for making half square triangles. Two squares are paired right sides together. Draw a diagonal line from corner to corner. Sew 1/4 away from the drawn line on both sides. Cut on the line that you drew. Hopefully it is square when carefully pressed open. I like to make the squares a little bigger , and then cut down the Half Square Triangle unit to the needed size--that way it is square.
I hope this helps you.
Rita