Cut out two 21″ x 21" squares of the main fabric, and use sewing pins to attach the “right” sides of the fabric together.
Pin along three of the edges. On the fourth side, pin only once at each end. The area without pins will be the spot where we insert the pillow form.
Sew along the edge of the pillow starting at the last pin before the large opening. Start with a few forward stitches, and then backtrack a few stitches to help stabilize the seam. Then, continue sewing the perimeter of the pillow until you reach the other side of the large opening. Make sure to backtrack again to finish the seam. Sew with a half inch seam allowance, removing pins as you go.
Trim the 4 corner points of the pillow at a diagonal, but be sure not to cut the seam. This will help form crisp, non-bulky pillow corners later.
Turn the pillow right side out, and iron the cover to remove wrinkles, and create crisp seams. With a ruler mark the dead center of each pillow front. Then, stuff with the pillow form.
Close up the pillow by folding the raw edges inward and pinning the two edges together. Thread a needle with coordinating thread and use a whip stitch to secure the two sides.