Here's where things get a little tricky! Trimming down the button PCBs and getting them mounted.
The buttons are reasonably simple devices, just a PCB contact patch and a conductive nub on an elastomer pad. They're common ground as well, which makes the wiring much easier. However, we've got to free them from the original motherboard, so that we can use them easily. This requires cutting the PCB with either very large pliers, or using a rotary tool. Large pliers reccommended, as cutting PCB with a rotary tool is unpleasant and creates nasty dust. The tradeoff is that cuts with a rotary tool are significantly cleaner, and there is less risk of damaging the traces by flexing them.
First, we have to safely remove the CCFL tube from the board. Snip the two leads on either end of the tube with your flush cutters, and the tube will be freed (along with it's diffuser). Please carefully dispose of the tube in accordance with your local laws.
The first tube lead is the copper strand coming out of that rubber bushing. The other is going to the trace marked "FL".
Next, you can fold the LCD out of the way, and begin lining up your cuts. Here are some diagrams for your cuts. You're going to be breaking a lot of components off the board, and generally destroying them... So, if you were planning on doing any salvage, now is the time. Time to meet.... the large pliers.
When you're all done, you should be left with two piles. One large pile of very sad electronics, and one pile of awesome button PCBs.
Now you can mount up these two PCBs with the screws that originally held in the motherboard. The boards will be sandwiched between the two halves of the case when it's all closed up, so don't worry about there only being a single screw holding the D-Pad PCB in place. It's really just to keep them aligned while the halves are separated. We'll wire 'em up soon enough.
Next, we want to add the capacitive pad for our Select button. It resides right above the D-Pad on the left side. In the above picture, you can see the yellow line indicating it's mounting point. It's reccomended that you solder on the three pickup wires (VCC, GND and OUT) just like the other two, so that you won't have to later. To avoid having screw heads popping out of the front of the case, this capacitive sensor will be glued down.
Use a drilling template for this one. This button will not mount exactly flush with the case unless you use your rotary tool to grind down the ridge that's there. Careful not to overdo it though, as it's very easy to cut through the case. Slowly grind away until the sensor board sits flat.
If you happen to have a spare elastomer pad from the 1, 2 or Start buttons, you can tack that down into the hole you drilled. It gives the Select button a stealth look. You may need to place a tiny bit of tin foil on the bottom of the nub to make it work a bit better, test it a bit before you completely finalize it.
Apply hot glue around the edges, to tack the board in place!