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.
Step 2: Disassembly
Step 3: Pre-Assembly
Measure a length(s) of the shielded wire long enough to easily reach from the pot mounting boss, to the jack hole(s) in the case, and carefully remove an inch or so of the outer jacket from both ends. Strip an eighth inch of insulation from both center conductors and twist the braided shield tightly. Tin the ends of all the wires and shields on both ends. Don't go bonkers. A little goes a long way. Next tin the lugs on the stereo jack(s).
Often, a volume pot has one leg soldered to the case and the case to ground, so that when you rotate the shaft, part of the signal is sent out this leg to ground to attenuate it. We don't want that to happen, so if there is a leg soldered to the case, unsolder it and bend it up even with the other two legs.
Clean up all three legs and add a fresh drop of solder to each. You could hook up the pot in parallel to two jacks mounted in the stock locations as I've done, enabling you to plug in from either side of the pedal, or just use one.
An easy way to hold everything in place while you solder, is to poke holes in a piece of cardboard, then stick the jack and pot shafts in them. Having said that, now solder the wires to the pot and jack as shown in the photo . Sleeve (shield) on the center lug, tip(red) on the left lug, and black (ring) on the right lug. Make sure you wire it the same on the jack, and it should look something like the photo.
Step 4: Assembly & Testing
Connect it with a stereo 1/4 inch patch cable to your pedalboard, and test and readjust to your liking, then put the cover back on. You could alternately test and adjust by "reading" the pot with an ohm meter connected between the tip and ring of a stereo patch cable plugged into the pedal. With most MIDI pedalboards, the actual values are not carved in stone. It's only looking at minimum and maximum values, and there is usually a provision to calibrate to a pedal.
Step 5: The Bottom Line
This instructable was condensed from the full article. Unfortunately, the site the article was on is gone now, but I will be republishing it soon and will add the link here