The best one is E. Burns method. For two and a half inch triangles cut out 2- 6 inch squares , place them right sides togather.. Draw an X from each corner and sew one forth inch to each side of lines. Measure 3 inch from corner and cut the square in half.
Then cut on drawn lines. You will have 8 half in sq triangles that are just a little large so you trim them to size and they are perfect. If you want 3 and a half inch use 8 inch squares. It is easy to figure what size squares to use for the size you need.