Well unfortunately I don't have my how to solder PSX tutorial up yet -__- too lazy to take the pictures, but it's VERY simple as long as you go with a cheap brand of PSX pad, such as the Pelican or Gamestop brands. There's a good explanation of grounds here,
http://www.darkravenwind.com/cdvision/madcatz.htm (click the wiring link on the bottom), for the positive side you have to wire each individually, but for the common side, you only need to wire to one ground, then daisy chain that ground.
Alright, if you're worried about which contacts are grounds and which contacts are positives, here's a REALLLLLy easy way to find out. Take off the top of your controller's case, so that you can see all the electronics, you can take the bottom off, but it's easier to handle this way. Get two moderately long pieces of wire, strip both ends of both wires. Now tie together one end of the wires. With the psx controller plugged into (I used a fighting games), place one of the exposed wire tips onto a ground. Place the other onto what you think is a positive, if the ground is really a ground, the positive will react, and the button will be pressed.
Example: I loaded up Capcom vs SNK 2, went into Training with the other controller, changed the 1st player to manual control, then had my 1st player PSX pad with the convertor plugged into the console, and exposed. I then placed one of my wire tips on the contact I knew was a ground, then I placed the other onto one I thought was a positive, nothing happened. A) I didn't have my other wire touching the ground well enough, B) the wire I touched WAS a ground, therefore there was no actual electric reaction occuring, hence nothing happened. Ok, so now I know that contact is a ground, which means the contact exactly next to it, usually divided by a line is the positive side, I place one tip on a known ground, then I place my other wire onto the contact I believe is a positive. Viola, a medium punch!! It's a positive, and I also found the ground.
On most controllers though you can actually trace the positive directly to the chip, and the ground has a green line going from one to the next, to the next.
-I didn't learn this from anything, this is just a method I learned from almost blowing out my controller with a multimeter (whoops)
-CthulhuLuke