Be careful your squares will not be on the straght of the grain.
A better way and just as simple is El Burns way.
This way your HST will be on the straight of grain.
For 2 1/2 in squares use two 6 inch sqs. Draw an x and sew 1/4 in on both sides of line. Then you cut the sq in half from both sides + then on the drawn lines. You will have 8 HST that just need a small cut to square up.
For 3 1/2 in use 8 in squares.