The second step is to remove the side plates that rest against your palms. This can be done with an iFixit opening tool or some iFixit opening picks.

If you have an opening tool, insert it near the tip as shown and "pry" the side plate away from the controller. You'll hear a click as the clips come loose. Slide the tool around the plate, prying as you encounter resistance, until it comes free.

If you only have opening picks, press them between the side and front plates. At first, you'll only be able to insert them in certain spots; push them in as deeply as you can until you have a few in place. Then, wiggle and slide the picks until you hear the clips loosen.