I have posted this before but its to good not to share again.
Using El Burns method.
For 2 1/2 inch half squares, put two 6 inch squares right sides togather. Draw and x corner to corner. Sew 1/4 inch on each side of line. Press.
Measure 3 inchs in from side and cut square in half, dont move fabric. Now measure 3 in on other side and cut in half, now cut between stiches. You will have 8 half sq triangle that only need to be squared up a tiny bit.
If you want 3 1/2 in use 8 in sq. 4 1/2 in use 10 inch sq. etc.