Measure the width of the top desk drawer where you want your writing shelf. Add 1 inch to the width. Cut your shelf board to that width using a table saw.
Measure the length of the inside of the drawer pulled out fully. Cut your shelf board that length.
Round the top edges of your shelf with the router bit and sand the edges smooth, first with coarse, then medium and fine sandpaper. Sand in the direction of the wood grain. Leave the lower edges straight.
Cut wood strips the same length as the shelf board. Glue them flush with the sides of the shelf boards. Test fit them on the drawer to make sure the top edges of the drawer fit between the two strips when the shelf board is set on top of the open drawer. Allow glued strips on the shelf assembly to dry overnight.
Apply wiping oil stain to the sanded and clean writing shelf assembly. Allow the stain to seep into the wood till the color matches the desktop and wipe off the excess. Apply several thin coats of varnish or polyurethane clearcoat. Sand between coats with very fine sandpaper.
Store the shelf near the desk. When you want a writing surface, pull out the top drawer and set the shelf board assembly on top. The wood strips will keep it firmly in place while you work. Some drawer compartments may even be large enough for you to leave it in place when you close the drawer.