Once the paint on the supports is dry, the table top can be mounted!
Drag in those big pieces of plywood and dry-fit them to make sure everything lines up OK. If your desk top is in multiple pieces, attach the largest piece first. Line it up flush against the wall. If possible, clamp it down to keep it from shifting while you work (or inevitably smack your head as you crawl underneath).
Tip: If you have clamps whose jaws can be removed, you can run the bar through a cable access hole and reattach the jaw on the other side of the desk surface. Then, clamp the desk top to the support beam!
The L-brackets are used to attach the desk top to the horizontal support beam. I spaced them between the cantilever supports. Start in the middle and work outward; first attaching the bracket to the support beam and then to the table top. You may want to use a center-punch to mark the holes and give the screw a nice little hole to dig into.
Once the L-brackets are in place, drive 2" screws through the cantilever support beam and into the desk top. Again, it's probably a good idea to pre-drill these holes with a countersink bit. I used two screws per support beam; one near the outside edge of the desk top and one near the wall.
The L-shaped desk that I built was made in two pieces. Unfortunately the two pieces were out of vertical alignment by about a quarter-inch. After attaching the two pieces to their individual support beams, I clamped them together to bring them into alignment. I then screwed on a pair of flat support braces to permanently hold the pieces together.
Well! Nearly done. The desk should easily support the weight of an adult or two. I proudly jumped up and down on the desk to prove it. So did my kids. I hope they don't think they're allowed to do this all the time...