Step 1: What You Need
For the electronics, you only need two parts: (1) a piezo transducer & (2) a 1/4" phone jack, both of which you can get for a few bucks at Radio Shack. You can probably order them online somewhere for cheaper too, or just pull them off of old electronics.
A piezo element is a sensor that measures force and converts it into an electric charge. That charge become the signal, sent through the phone jack, which is sent to the drum machine or interface.
Tools you will need include: bandsaw or handsaw, drill press, drill bits, boring drill bit, soldering iron, epoxy glue, screw driver and ruler.
Step 2: Cut Out a Piece of Wood and Drill a Space for the Piezo Disc
If you have a 2x4 you could probably cut your trigger on a band saw or miter saw. With that big chunk of wood, I used a hand saw and a circular saw.
Once you have cut the piece you want, sand it down so its smooth to avoid getting splinters or cracking the wood.
Then find the center of one face of the piece and drill a hole with a boring drill, one that's large enough for the piezo element to fit inside. This way we can hide it in the piece of wood. You can usually leave the piezo element in the plastic case it comes in, it will still work fine, but if you need to conserve space, you could crack the case with a flat head screw driver and simply clue the piezo disc itself to your trigger.
Measure your piezo element and then use a boring bit or smooth finish bit to make a hole that will fit the piezo element. The largest boring bit I could find at my school's shop was 1.25", which wasn't quite big enough for the piezo element, so I drilled little ears on each side with a 1/4" bit so it would fit.
Step 3: Place Piezo Element
Step 4: Install the 1/4" Phone Jack
First, measure the diameter of the phone jack. The one I used was 15/32". Find a drill bit that will make a hole that is as close to that size as possible and screw into the wood block, just behind the hole where the piezo disc will sit. Then connect the two holes by screwing underneath where the piezo disc will sit.
Once you have done this, there should be a nice little tunnel to but the phone jack in, which connects to the area where the piezo disc sits. This is just one way to set up the electronics, but I've found that it works pretty well. Once you see that each piece fits well, place the piezo disc into place while threading the two wires, the red and black leads, through the hole.
This is a good time to test your electronics, before you solder or glue anything. Attach alligator clips from the red and black wires from the piezo element to the two terminals on the phone jack. If you then plug a guitar cable of 1/4" cable into the jack and then into your drum machine or interface, you should see a response when you hit the piezo element or the wood block. The red light dot on my Alesis drum trigger interface in the pictures above indicates that it is getting a signal. This should work with any drum machine or interface that takes 1/4" cables from triggers.
After you test to make sure that signal is connecting, remove the alligator clips and solder the red and black wires to the terminals on the phone jack. Because the trigger sends just an electrical signal and not audio, it doesn't matter which wire is soldered to which terminal. It is also a good idea to use mono phone jacks. Stereo jacks will work, but you have to make sure to solder to the correct terminals. Using mono jacks will help you avoid that potential mistake.
Step 5: Finish Up!
After that, put a thin layer of epoxy onto the outside of the phone jack and push it into its hole. The glue will help it stay in place when you start putting cables in and then pulling them out when using the trigger. Wait for the glue to dry and then your trigger is read to go!
If you want to make the trigger even more convenient to play with an electronic drum set or any other musical set up, you could buy a mounting bracket like this one from Latin Percussion and screw a hole into the trigger to hold the bracket. There are many different ways to mount a trigger like this one with hardware from a percussion company like LP or your local hardware store.