The Half-Square Triangle quilt block (also known as HST) is another basic building block for a ton of blocks. The math behind how to figure out how big to cut the squares is pretty easy. If your finished patch is to be 4” square, cut the squares at least 4⅞” square. I prefer to cut the squares at 5”, so that I have more wiggle room when trimming the resulting patch to 4.5” square. So a 2” finished patch would start with two 2⅞” or 3” squares, etc.
Match the two fabric squares right sides together. Mark a diagonal line from corner to corner on the back of one of the squares and sew 1/4" (or a little less) on each side of the line.
Cut along the diagonal line. You will have 2 half-square triangle patches.
Open up the patch and press the seam allowance toward the darker fabric.
Place the diagonal line on a square up ruler along the seam line of the patch with a little bit of fabric showing around all 4 sides. Trim the right and top sides with a rotary cutter.
Lift the ruler up, turn the patch 180 degrees, and line up the diagonal line of the square up ruler with the seam line. This time match up the left and bottom sides of the ruler with the left and bottom sides of the patch and trim the right and top sides of the patch with a rotary cutter.