There are lots of ways to make HSTs, but whichever way you use, don't try and cut the pieces to size before you sew. Cut them larger, then trim them down. When you trim, make sure that the seam is exactly on the 45 degree diagonal. It should go precisely thru the corner. The normal method is to start with squares 7/8" larger than your target size. Most folks round up to 1"' but if you are a newish quilter, I would make them even larger, adding a quarter inch or more. As your skills improve, you can scale this back. Hst tools are great, and I second the mention of the bloc loc rulers. A bit pricey, but worth it to me