Start with
2 squares at 5.5" square in your pinwheel color
and 2 squares at 5.5" square in your background color.
Layer these in two pairs;
mark the diagonal line;
sew 1/4" off the marked line (on BOTH sides of the line);
press to set; cut apart on the drawn line;
press open to have 4 HST squares -- which you will then trim to 4.5" square.
Sew together to make you 8" (finished pinwheel block.
Jan in VA