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!