We now focus our attention on the ZL and ZR shoulder buttons inside the rear panel assembly.

Push the tab for each on sticking straight up towards the top of the rear case and thr button should fall out.

And now for the bigger, plain ol' L and R shoulder buttons. Remove the two silver screws holding the shoulder button assemblies to the rear panel.

Push down on the each button, then pull the black part of it towards you to free them from the rear panel.