Not as hard as it might sound.  Each button needs two wires.  A ground and a wire running back to an input on the KeyWiz.

I got a bunch of 16 gauge wire in as many different colors as I could get.  A friend had a bunch on spools and I cut at 3 foot lengths.  I planned to mount the KeyWiz right below the control panel so 3 foot per was plenty.  I got a bunch of black for the ground.  I also got a bag of female disconnects that didn't have any insulation on them.  I planned to solder the wire to the disconnects which would then be pressed onto the micro switches.
 

With a wire coming from each switch back down neatly to the wire harness, I cut the wire to an even length.  I took my time here.  I wanted this to be clean and manageable in case I had to move wires.  I also wrote down what each wire color was and which button it belongs to so I could later connect it to the KeyWiz.

The KeyWiz was a great time and headache saver.  Only 35 bucks.  What this cool gadget does is allow you to switch from a regular keyboard over to the inputs from your buttons.  Each input is labeled and it is setup with all the MAME defaults.  The wires from each button get screwed down into the industrial quality terminal block on the KeyWiz board.  A PS2 cable is included and that runs from the KeyWiz to your PC.  You then connect your keyboard into the KeyWiz.  Flip the switch and you can use your keyboard or use your buttons.  It's also programmable for custom jobs and has other features like something they call  Shazaaam key which allows 24 keys to have alternate assignments.  There's a bunch more features to this KeyWiz board.  I used the defaults.  Very easy.  Forget the keyboard hack.

Following my sheet for the keys I was using and their default key mapping to which terminal block, I wired up the KeyWiz to my controls.

The KeyWiz switch needed to be accessible.  I chose to mount it inside the coin door, right below the control panel.  The wire harness drops down, makes a loop as in the picture.  I screwed the KeyWiz to the cabinet using the spacers they provided with the KeyWiz.  Here's a tip: Use some chewing gum to hold the spacers to the screws while you mount it.  I hate dropping screws.

Before I started permanently mounting things, I wanted to test my control panel.  So I connected up my computer (more on the PC later), and an old CRT monitor (LCD was still on order) and I tested to controls.  Surprisingly, it worked.  Except for the fact that I wired my joystick backwards.  But it works!  Woo Hoo!

The first game?  Well that would be Astro Invader of course!

My plan was to have the keyboard accessible from the coin door if needed and a USB trackball mouse connected to a USB hub mounted on top of the game.  I have the computer booting up and starting the MAME Emulator program automatically (shortcut in the startup folder).  More about all that later where I'll go over the computer itself in detail.