Use the phillips head screwdriver to remove the 6 screws from the rear of the controller and set aside the back casing.
Now you can remove the circuit board. If necessary, use a dry rag to clean the contact points on the circuit board.
Remove the old button pads from the front of the controllers casing and replace them with the new pads.
Finally, you can fit the circuit board to the front controller casing and re-fit and secure the rear controller casing with the 6 Phillips head screws. Your controller should now be working as good as if it were new!