It's very easy, you just need a Torx screwdriver, a drill (to widen the holes in the bottom of the metal case to fit big screws), 5mm thick, 10 to 12mm long machine screws (8 of them), and some washers to space the board up off of the case (plastic ones to touch the board, metal ones above and below those).
You'll also need thermal paste, as it overheats if you don't put any on, but after I put a drop of Arctic Silver 5 on the chips, it ran fine. The point of this mod is that tightening the heatsinks presses the chips against the board, creating a more solid connection than the looser, spring based X clamps. If it doesn't work, you can try tightening the screws even more, but be careful not to overtighten, as it can break the chips and/or the board.