Screw them together. See the Sketchup plan (attached). The 48" boards are obviously the "rails" and support posts. 20.75" boards should link the rails together to form front and back panels. Squaring things up may be a challenge,...
Add your gear. Use more wood screws to secure faceplates to the wooden posts. For small items like cable modems and wireless thingamajigs, you can put wood screws into the side and hang them (most of these have special slots, as if they were......