1. With wrong side inside, fold each backing 21x25" rectangle in half to form two double-thick 12-1⁄2x21" rectangles. (The double thickness makes the pillow back more stable.) Overlap folded edges about 4" to make a 21" square rectangle (Pillow Back Assembly Diagram). Baste across top and bottom of overlapped layers to make pillow back.
2. Layer pillow top and pillow back with right sides together; pin. Using a 1⁄2" seam allowance, sew together around outer edges to make pillow cover. Turn right side out; press. Insert 20" square pillow form through opening to complete pillow.