your Computer Desk Rightside Up
Turn it upright now.
Put two screws on each side through the top into the 2x4's that are attached to the legs. It takes some measuring to figure out the placement. After the monitor shelf is added, you won't be able to see these screw heads.
Here's an upright full view.
I cleaned the background off these pictures to make the construction more clear. If I had left it all there, you would see a small and very crowded dining room