Step 1: What You Need
Of course you'll need a wah-wah pedal. I used an original Vox wah from the 1970's. They make a reissue of this pedal that you can buy new for $100 or less. A Dunlop Cry Baby wah is basically the same housing so it will work just as well, and is even cheaper. I've seen them go for as llttle as $20 on eBay and in the pawn shops.
The potentiometer is the toughest item to find. I already had a suitable pot, but I imagine Radio Shack might be a source. You need a 10K ohm pot, such as used for volume/tone controls. The only problem you may have, is finding one with a long enough threaded shaft. It has to be long enough to span the mounting boss in the case with enough protruding to attach washers and nut. Use the stock pot as your guide.
You'll need one or two 1/4 inch stereo jacks, depending on which configuration you choose. Basically you can have just one input jack on one side of the pedal, or you can have an input on either side of the pedal utilizing the stock input/output jack locations. This makes more sense to me, and is how I did it, but you really don't need two jacks. Again, Radio Shack is your friend.
To wire it all up, we want to use shielded two conductor audio cable. You don't need much, probably a foot will be more than enough. I cut what I needed from an old mic cord. Of course you'll need a standard stereo 1/4 inch patch cable to plug the pedal into your controller.
Basic hand tools; screwdrivers, wrenches and/or sockets, safety glasses.
Soldering iron/gun and 60/40 solder.