I cut squares that are exactly 7/8 bigger than the finished size needed, draw the diagonal line on the light square with a brown Pigma pen (it has a fine tip and won't run), then draw a quarter inch line on each side of the diagonal line. After putting the 2 squares right sides together and sewing exactly on the quarter inch lines, cut on the diagonal line, and have 2 perfect HST's.
I know I should be able to just sew a quarter inch on each side of the line without marking, but it's never worked for me.