Cut out a piece of fabric that is 18 inches wide and 24 inches long. Lay this fabric piece right side down on top of a table.
Pile some cotton batting on top of the fabric so that it is about 2 inches deep and takes up about 12 by 18 inches of the fabric in the center, leaving the edges clear.
Place a piece of 12 by 18 inch plywood directly on top of the quilt batting, centering it in the middle of the fabric piece.
Pull the edges of the fabric tightly over the edges of the plywood and hot glue the edges of the fabric to the top edges of the plywood. This first piece of plywood will give shape to the lap desk by allowing the fabric to stretch over it.
Hot glue another 12 by 18 inch piece of plywood directly on top of the first piece of plywood. This second piece of plywood will cover the raw edges of the fabric that were glued to the top of the first piece of plywood and allow for a smooth top to the lap desk. The cushion made by the fabric and batting will rest on your lap for comfort while you use the lap desk.