Step 1: Buy a cheap toy guitar
While you are doing this, download the free Guitar Hero clone Frets On Fire. You can also find on the web all the songs from Guitar Hero I and II! All for free!
Step 2: Hack a keyboard
The trick is track down the traces for 7 keys (14 wires). Though you can use 7 keys of your choice, I recommend you trace 7 keys that are on the same trace (this changes the number of required wires from 14 to 8). Be careful, some cheap keyboards prevent you from pressing certain combinaisons of keys, so 2 or 3 fingers chords might not work.
Controls to be used with the guitar:
Button 1, 2, 3, 4 and 5;
Flipper Up and Down.
I bypassed keys 1, 2, 3, 4, 5, X and Z, by soldering some extra wires to the parallel port connector.
Cut one or two RJ-45 cables (LAN) of about 10 feet long, and solder the wires to the male connector.
Step 3: Rip open the toy guitar
Bring your RJ-45 wires from the keyboard to the neck circuit. With a knife, cut the tracing and solder your own wires for the first five buttons.