Step 1: Obtain the nessisary parts for the guitar
1: A Guitar. Any kid's toy guitar will do, though one with fret buttons will make things easier and nicer looking. Otherwise you will need to add buttons for fretting. Mine came from Wal-Mart for 10 dollars.
2: A keypad. I got a cheap USB numberpad while i was getting the guitar for about 12.99. I prefer USB, and the smaller number of keys makes deciphering the buttons easier. We will be using the controller board in this to connect to the compuer.
3: Switches and other parts. The amount of switches you will need may vary depending on your choice of guitar and if you want to navigate the game menus without a keyboard. The game itself supports 5 fret keys, one (or two) strum key(s), escape, and 4 directional buttons. a leaf switch will work well for the strum key. I also used some prototyping circuit board and a spool of very thin wire to keep things straight, you may be able to do without these depending on your skill level. These can be sourced from sites like digi-key or your local rat-shack.
Step 2: Dissassemble and remove unneeded parts
First i unscrew and take the back off of the guitar, remove the logic board, speaker, and dial. I left the long "fret board" in place since i will modify it to work with the keypad.
Next is to disassemble the keypad. Mine only had two screws outside and two inside holding it together. What we want here is the circuit board which is the brains of the keypad. you want to keep the plastic sheets with traces on them (also known as the key matrix), as they can be useful in figuring out the keys.