Step 4: Cut & Mount the Horizontal Support
I first added a bit of a bevel to the ends of the supports with a miter saw. This isn't necessary, but it looks nice.
With the help of a friend, mount the support to the wall using a 3" screw, in the middle of the support. The support should now be able to pivot on this screw. Place the bubble level in the center of the support, and align it perfectly. Then, add screws to each end of the support, again to studs. I used two 3" screws at each intersection (ie. two screws every 16 inches).
If the desk is being build into a corner, clamp the second support onto the first to make sure they stay aligned where they meet. Screw in the second support piece just like the first.