With the screws removed, turn the controller upside down. Insert the iFixit opening tool between the top and bottom casing to pry them apart.

Separate the top and bottom casing.

It is likely that the buttons will fall out of place, so as you open the controller, try to keep track of where the pieces go.

There should be the four circular buttons labeled 1-4, four smaller circular buttons for the D-Pad, and three rounded rectangular buttons.