Step 2: Marking the Side Panel
You'll want to be aware of how the monitor will appear once mounted. Will there be enough room for cabling, keyboards, etc.
I then center punched each mark with a nail, then drilled through the panel. If you haven't drilled through metal before (and even if you have), you'll really want to do this. Drill bits tend to move all your work piece - punching them beforehand leaves a divit in the metal surface, which will help prevent the drill bit from 'walking' while in use.