1. Use a pencil to mark a diagonal line on wrong side of each solid white 3-3⁄4" square (Diagram 1).
2. Layer a marked solid white 3-3⁄4" square atop a solid red 3-3⁄4" square. Sew together with two seams, stitching 1⁄4" on each side of drawn line (Diagram 1). Cut apart on drawn line to make two triangle units. Press each triangle unit open, pressing seams toward solid red triangles, to make two red triangle-squares. Each should be 3-3⁄8" square including seam allowances. Repeat to make 128 red triangle-squares total.
3. Referring to Diagram 2, sew together four red triangles-squares in pairs. Press seams in opposite directions. Join pairs to make a red pinwheel unit; press seam in one direction. The red pinwheel unit should be 6-1⁄4" square including seam allowances. Repeat to make 32 red pinwheel units total.
4. Using remaining marked solid white squares and solid blue squares, repeat steps 2–3 to make 32 blue pinwheel units total.
5. Referring to Diagram 3, sew together two red pinwheel units and two blue pinwheel units in pairs. Press seams toward blue pinwheel units. Join pairs to make a Pinwheel block; press seams in one direction. The Pinwheel block should be 12" square including seam allowances. Repeat to make 16 Pinwheel blocks total.