Originally Posted by pocoellie

Here's how I do HST, if you have a bunch of them to do, there's a grid method of doing them. Instead of 7/8" of an inch, I use 1", then square them up.

The magic number for half-square triangles is 7/8. Simply add 7/8 of an inch to the desired FINISHED SIZE of your half-square triangle. For example, if I want a 2" finished size half-square triangle, I will cut two squares (one from each color) that measure 2-7/8" each. There are two methods that you can use to construct your half-square triangles from squares. Either way, you'll end up with two half-square triangles.
Method 1 - Cut Then Sew: With your rotary cutter and ruler, slice each square from corner to corner to make two triangles of each color. Layer color 1 on color 2 with right sides together and stitch a 1/4" seam along the long side. Repeat with the second pair of triangles. Press seam toward the darker fabric and trim points. You now have two half-square triangles that measure 2-1/2" on all sides. They will measure 2" when sewn into the block and then into the quilt. This method works fine, but you do have to handle stretchy bias edges. If you use this method and your half-square triangles turn out to be not exactly the right size, you might want to give Method 2 (below) a try.

Method 2 - Sew Then Cut: With a pencil and ruler, draw a line on the wrong side of the light color square. With right sides together, layer the lighter square on the darker square and stitch a 1/4" seam on BOTH SIDES of the drawn line. Cut apart on the drawn line and press seam allowances toward the darker fabric. Trim points and you are done. You now have two half-square triangles that measure 2-1/2" on all sides. They will measure 2" when sewn into the block and then into the quilt. This method lets you avoid handling the bias edges, therefore yielding half-square triangles that are more accurate in size.