Cut two squares 1" larger than finished hst. Right sides together. Draw a diagonal line from corner to corner. Sew 1/4" from each side of the line. Cut on line.
I have Eleanor Burns ruler where you trim before you press open. Put the line on the ruler along the stitching line and trim two raw edges.
If you don't have the Eleanor Burns ruler then press open and put a square ruler with the 45 degree line along the seam line and trim two sides. Turn, repeat to trim the other two sides. If you just trim two sides and not have the ruler line along the seam line your hst will be lopsided.