Okay, for a while I worked the rounds at a Bemani board that saw a lot of newbies and a lot of PSX controller hacking (to make DDR pads). I got absolutely sick of requests to edit the photos to show where the solder points are because, if you're smart enough to solder to the controller and make the DDR pad / control panel to go with it then you are smart enough to figure out solder points yourself and will be the more electronically-aware for it.
Realization 1 : each button area has two sides, and when these two sides are electrically connected, the button is considered to be pressed.
From this realization you already have enough to know where to solder to because those contact pads are big, clean and copper. But there's another thing that will help:
Realization 2 : each button area has a side that is electrically connected to a certain side of every other button area. This is called Common (COM) or Ground because it is common to every button area, and 0 volts (ground level, so to speak).
That was a little extra information just to help with your electrical knowledge, but the gist of that realization was that you only need to solder 1 wire for the ground side of every button.
Summary - solder 1 ground wire, and 1 wire to the OTHER side of every button. Thus is your controller hacked.
Oh, and that Yenox microswitch looks pretty compatible with the more popular Cherry microswitches, but of course I'm not very knowledgable in that.