You need: fabric; pillow form; sewing supplies.
Cutting: Measure dimensions of pillow form. Add 3 1/2" all around for flange. Cut two pieces to obtained dimensions for pillow front and back.
Sewing: With right sides together and using 1/4" seam allowance, sew front and back together on three sides. Clip corners and turn right side out. Fold remaining edges 1/4" to wrong side and press. Mark dimensions of pillow in center of cover. Topstitch along marks leaving one side, same side as opening, unstitched. Insert pillow form and topstitch along center remaining side. Slipstitch remaining open side closed.