you can remove those 2 solder button boards, they only give a platform for the buttons to press against.. since you're hacking it and just using wires this is'nt needed.
carefully desolder the wires running to the main boards, usually there will be 3 wires, although sometimes more, on these particular pads there is only 3.
if memory servs the center is ground and on either side is one of the buttons.
once you've desoldered htem you can run your wire though the holes on the main board.. this makes for easy and strong joint.
this is where i tapped into the ground.. you can do one side or you can do both sides if you wanted 2 grounds.. but you only need 1 ground.
as for cleaning the board, some fine sand paper.. i suggest doing this by hand and not a dremel.. infact you can probably get by with just rubbing alcohol..
anyway take some fine sand paper.. say 500+ rub it a few times.. be gentle you only need a little.. then clean it with alochol.. then put some solder on the traces like you're tinning wire.. you just need a dab to gain a foot hold on the board.
then tin your wire and solder it to the tab thats on the board.
what i did was then use zip ties to try and secure the wires to the board.. i wrapped the wire around the zip tie so that they would'nt easily be yanked off the board..
then i used the zip ties as mounts of sort and put staples though them mounted to a piece of scrap MDF.. they're not sold on there but they're not gonna go anywhere.
the pads i was working had a cap on the back which prohibited flush mounted with screws.
i plan to finish it up with some terminal block's
as for the D-pad these ones are matrix as some one pointed out..
you can't have an axis without a + and a -, there for it can only be one or the other at a time.
what you do is solder either up or down, and left or right.. you only need to do this 1 wire per axis.
next follow the circuit you'll see one side is linked to common ground for all other buttons.. and 1 side is linked to a secondary ground, it may possibly be the same ground with a resistor on it, as iirc this secondary ground is 2.5v while the common ground is 5v
i had not tested with a resistor but why bother when it's already done on the board.
there is a pro and con to this.
the pro is you only need 14 wires for the whole pad, you have 2 grounds but also only need 2 wires for directional pad.
the con is your forced to use the dpad as either the joy stick which can't acuate more then 2 of those directions at a time.. or asign them to buttons that wont be pressed at the same time.
for most uses this is not a problem, since most will have 4 or 8 ways which this situation can never arise.
all and all these are still great pads for hacking.
a little tip when it doubt break out your multi meter and set it to DC 20 or just DC if it's auto ranging.
on these the positive side is the one that has a circular spot in the trace close to the button.
thats what makes these great because it's very easy to solder to, just connect to to ground and voila you got a button press.