How To Make A Half Square Triangle Take two squares of the same size. They should be 1/2” larger than you want your trimmed HST. Mark a line from corner to corner on the back side of one square. Sew 1/4” away from one side of the line. Flip it around and do the same on the other side. Cut along the marked line. Press open with the seam folded toward the darker fabric. That’s it! Each square pair gives you two identical HSTs.