The easiest way I've found to do it is:
Whatever your rectangle is.....say 6 1/2" x 3 1/2".......cut your squares the 3 1/2" measurement. Draw a diagonal line across your square......then a 1/2" outside that. Sew on the drawn line and the 1/2" line, cut apart, press. Do the same to the other side. This gives you a perfect flying geese unit and 2 hsts. I just did that to the civil war quilt I just posted and I already have all the hst all put together for a border.