If you are working with square seat cushions you will want to make sure your patterns goes up the sides. Pin the towel into place and cut off excess fabric leaving a one-inch seam allowance. Remember you will be using a solid colored (less expensive towel) on the back side of your cushions.
Extra-large beach towels should be long enough to make one back pillow. Sew the beach towel into a large tube so that the seam is along the back and the design is centered on the front. Sew the towel closed at one end and add Velcro at the other. Stuff the slipcovers with two king-sized extra firm pillows.