After cutting 8 fat 1/4's into strips of varying widths, you sew them into 4 stratas. Then, you use a 9 degree wedge ruler (you can also use a 10 degree, which will give you a larger final piece) to cut wedges, largest first. The first one is just under 19", the smallest, less than 1". The you start sewing the wedges together, smallest first, from each end. When you get to the last wedge, you have the two halves and sew them together at the widest point.
Kind of hard to describe, but not hard to do!