This is my first instructable and I plan on entering it into the "Get the Led Out!" contest. Constructive criticism is always welcomed, Also if you find this guide useful or just enjoy it, please vote and rate it.
*** Updated with circuit to make it blink when you strum! Also how to make the leds turn on when you plug in your instrument cable! See the last step!
http://www.youtube.com/watch?v=JtR5kkf7ipw heres the link to it in Action! skip to 6:00 if you don't want a video tutorial!
****There is another method of doing this that hsandford has brought up, It is a bit more complicated, but will result in much better results and have less problems overall then this method. Your can find his guide here. http://howardsandford.com/blog/flashing-leds-audio-meter-avr-attiny-guitar-pickup-wiring-sound-following-circuit/
Step 1: Materials.
1. 8x led, color of your own choice. (you can add more but they might not fit correctly in your guitar.
2. 2x resistors, make sure you have the right kind for your led.
3. pieces of wire.
4. Solder and soldering iron, other electrical tools, wire cutter, wire stripper etc.
5. 9v battery and 9v battery clip.
6. 9v battery holder, or way to hold it in place.
7. tools to remove guitar strings and pickups.
8. plastic that is semi transparent (I'll show how to make this in case you don't have anything suitable), used to diffuse the light for a better glow. a dremel tool may be needed for this, also scotch tape.
9. Extra strings in case something happens to yours, even though its very unlikely.
10. You will also need a switch of your choosing, I used a reed switch that is normally on so i can just stick a magnet to it to have it off when I'm not playing. Otherwise you will need to drill the plastic electrical cover in your guitar.
11. basic soldering and electrical knowledge.
** optional, If you are doing the led blinking version, you will need,
1. LM386n-1 op amp chip, radioshack has them for 2 dollars.
2. A 10 uF capacitor
3. various resistors to get the desired amount of blinking.