Remove the 6 x T-8 Security Torx Screws holding the front and back together. The Red circled screw will have a white sticker over it, if the sticker’s missing, someone else has already peeked inside—maybe looking for buried treasure.

Good news! All the screws are the same length and you do not need to remember positions. Once the screws have been removed you will be able to lever near the bottom of the grips to remove the back of the controller (red arrow)