Measure the pillow form. From one fabric cut a strip 3 inches wide and long enough to equal the sum of the four sides of the pillow plus enough for seams. You may have to piece your strip. Fold in half the long way with the wrong side inside and press. Machine-stitch close to the cut edges, pleating fabric as you sew.
From the remaining fabric, cut two pieces the dimensions of the pillow form plus enough for seams. Layer the two pieces with right sides together. Place the pleated trim on top of the layers with raw edges aligned. Sew through all layers, leaving an opening along one edge, to make the pillow cover. Turn pillow cover right side out through opening. Insert pillow form; stitch opening closed.