Remove the six T-8 Security screws holding the front and the back of the controller together. Note: one (highlighted green) is covered by a white paste/sticker that crumbles when removed; this lets you know if someone has previously disassembled your device.

Once the screws are removed, you can insert a spudger under a vibration motor to provide a little leverage to separate the front from the back of the device.

The third image shows the front assembly removed from the back cover.