Step 1: Measure your bookshelf, and procure your materials.
Our bookshelf is 31 x 15 1/2 inches. So we procured some woods with which to build a tray that fits on it.
Our base is made of 1/2 think pine board, cut to a 32" x 16" rectangle - a wee bit bigger than the top of our bookshelf, so as to make for a not-too-tight fit.
The walls of our tray are made from 1/2" by 6" pine planks. Two six foot long pieces were more than enough.
We used some handy dandy corner braces that we got from the Home Despot (see pic), along with some wood glue and some longer skinny wood screws suitable for screwing the planks in at the corners.
Even though we are prejudiced against hippies, we still act like them sometimes. So we use natural untreated woods.