I first made this instrument a few years ago and have been meaning to put up a guide ever since. This won't be a step-by-step 'do this exactly' type instructable. I'm going to show you how I did it, highlight the important parts and let you realise you could have done this all by yourself if you really wanted to.
You Will Need (roughly):
Wooden Tennis Racket
A Piezo Transducer (http://www.maplin.co.uk/piezo-transducers-3202)
A 1/4” Mono Chassis Socket (http://www.maplin.co.uk/1-4-mono-chassis-socket-1252)
3 Tuning Pins (I got mine here: http://windworld.com/products-page/hardware-for-acoustic/zither-tuning-pins/ although if you want to go proper DIY, check out my post on DIY tuning pins here: http://www.vulpestruments.com/2013/02/a-note-on-diy-tuning-pegs.html)
Some scrap wood
A few bolts
Step 1: How to Make a Contact Mic
With this simple little thing you can turn just about anything into a plug-in-able noise making monster.
The piezo works by taking small vibrations and converting that into a small AC-Current via crystals and magic. This, when amplified, makes loud noises.
A little tip for this, I often find the soldering on the white part of the piezo to be quite fragile. I usually place a bit of cellotape ontop of it to help strengthen it. It seems to work!
Step 2: Mounting the Piezo
You can decorate the wood and stain it and make it all nice and pretty if you wish. I didn't just for the purposes of this demonstration.
Next up, wriggle the mono-socket through the wire on the racket and add the washer/bolt to hold it in place.
Measure up and drill 3 holes into your scrap wood close to the piezo but not so close that you'll drill through it. That would be bad.
Step 3: The Brackets
I then bolted the L-Brackets onto the tennis racket and through the wood. I had to wriggle the tennis rack strings around a decent amount to get them to fit.
It would be better, if instead of 3 seperate L brackets there was one solid piece with 3 holes drilled into it. I didn't want to over-complicate the instructable and have people drilling through metal if they weren't comofrtable with it. The reason this would be better is that it would be stronger under the tension of the wires and not warp when you tune the strings up.
Step 4: Holes and Holes
Close to the handle I drilled 3 small holes like in the picture.
I also drilled 3 holes on the opposite side to where the L-brackets stick up. These 3 holes are for the zither pins (drill size 4.5). Again, if you want other tuning options, go for it. There are plently.
I then added the tuning pins and was ready to wire up!
Step 5: Wiring Up
I used the bolts as anchors and added another bolt ontop to secure the wire (probably didn't need it but better safe than sorry). Then through the L bracket, through the neck to the back and into the tuning pins.
This through the neck method cuts out a few little headaches when making instruments quickly.
Step 6: Make a Racket!
There are loads and loads and loads of different ways to achieve the same effect. I've thought up ways to include frets but haven't found the time to play with that idea just yet.