Al,
Yes drill before installing. You can use one of the metal sides to calculate where to drill.
I bought three sets last wednesday in Ikea and they were on sale for $24.00.
These are the best pics I could took without having to clear and move the detolf.
Finally a not so good pic of the top light wire getting into the electrical tape that runs from the top to the bottom of the plastic corner keeping all the wires together and hidden.
Hope these clarify a little more! Good luck!