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.
Step 4: Create the flipper
(I used some sort of plastic piece usually found in the back of drawers to hold them on a track, but I'm sure you can figure out somethin ;-)
Step 5: Create the flipper circuit
Use the existing screws to run springs or rubber bands to go away from the flipper. I used two screws placed vertically in the legs to hold the springs. This will have the flipper come back to a neutral position when released.
Hot glue a long thin piece of metal to one of the legs of the flipper. Glue a connector under the tip of the piece of metal and another connector above it. When pressing the flipper down, one connection is made. When pulling the flipper up, the other connection is made. (My second connector is placed on the speaker's socket)
Run a wire for each part. Make sure the keys used for Up and Down are on the same trace in the keyboard. You must have only 3 wires in this part of the circuit.
Step 6: Wrap up everything
Cut a hole in the bottom of the case to run the cables out to the keyboard (if you did something like me) or to the computer. I made a knot with the wires inside the case so the wires are not accidentally ripped off.
Step 7: Get a grip
The strap is a nice add on so you can play while standing up!
Launch Frets On Fire, connect the guitar, setup FOF so the key settings reflect those of your guitar, and rock on !
(Side note, plan a Esc button on the guitar, otherwise you need the keyboard to be close by)
I didn't bother making this wireless, but I'm sure someone is crazy enough to use a wireless keyboard !!! :-)