This was Martha's quick tip for February, I've done this before and it works great. I was always guessing about the size but with this info, no more guessing. Great tip!
A quick and simple way to make half-square triangles
1. Layer 2 squares of fabric right sides together and pin.
2. Sew ¼" seam around the entire square, stopping ¼" from each corner and pivoting with the needle down to sew the adjacent side.
3. Line up a ruler corner to opposite corner and cut the square diagonally. Being careful not to move the square, now cut diagonally in the opposite direction.
4. Open each of the four squares, press seam allowances toward the darker fabric, and trim the dog ears. Edges on these units are on the bias, so be careful not to stretch them as you press and sew.
How big should the squares be? Whatever size FINISHED half square triangle you want, add TWO inches to that measurement. For example, if you want a 2 ½” half square triangle (finished size 2”),you would cut your squares 4”.
2” finished – 2 ½” unfinished = 4” squares
3” finished – 3 ½” unfinished = 5” squares
4” finished –4 ½” unfinished = 6” squares
5” finished – 5 ½” unfinished = 7” squares
Make one set, cut them and measure to be sure your quarter inch seam is correct and they measure exactly what you want.
If you spray starch these or use Best Press by Mary Ellen, there should be no problem with bias edges or cut your squares to begin with on the diagonal. You will have bias edges one way or another, so just be careful and don't stretch.