After you have unscrewed the four Phillips head screws on the back of the controller (see other guides), you will need to unlatch a total of 4 clips holding the casing together before you can open the controller

The clips are nearly impossible to see from outside the controller before disassembling, so the pictures show the assembled and disassembled casing to help you find them

Start with the two clips located on either side of the ext/headphone port; note that the photo shows the casing after the clips have been unlatched, so your gap will not look that large yet

To unlatch each clip, insert a spudger, thin flathead screw driver, or other thin object that can fit into the gap between the top and bottom casings at the locations indicated with arrows

Push your tool towards the interior of the controller while gently twisting or levering it to further open the gap and separate the top and bottom casing