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.
Step 3: Prepare parts
The keypad controller i used has a black carbon coating on it's traces. To remove this I carefully scraped it off with a screwdriver, then once most of the contacts were exposed i used rubbing alcohol to clean off the remnants. I then tinned each of the 12 contacts on my controller board with solder.
Next is the actual case of the guitar. Frets on Fire requires buttons for navigation, cancel, and strumming, so I added four small pushbuttons in the upper body of the guitar, a leaf micro switch for strumming, and a button where the volume dial was as a back button. The back button's recessed fit makes it hard to hit accidentally.
I also had to modify the fret board since it ran all the keys off a single ground. This required me to cut the trace linking the buttons together, strip back the green masking on the circuit board, and solder new wires to the freshly exposed circuit trace for each button. I then found it's matching wire at the bottom of the board and twisted them together to keep from losing track of which pairs match up.
Next I painted the fret keys. The normal color pattern for a guitar hero type controller is green-red-yellow-blue-orange. Make sure you know which buttons on your guitar fit which holes in the neck. I didn't notice that each of the buttons on my guitar are of slightly different sizes until after I'd painted them.
Step 4: Wire it up
Then i found a good spot for the controller+breakout board and used a small amount of hot glue to hold them in place temporarily.
Then I added a pair of wires to each button. These will be connected to the breakout board. Below you can see my setup with the wiring finished, but we're not quite to that point yet.
Step 5: Deciphering the Key Matrix
If you look at the plastic contact sheets from the keypad, you can see it consists of two contacting layers. When testing for connections you should keep one side of your testing wire on a contact from one layer, while probing contacts from the other layer of contact sheet. On mine pins 1-6 were on one sheet, while pins 7-12 were on the other. This made my method of testing something like the following:
1. connect a wire to pin 1
2. set up excel sheet to record results
3. open notepad to test with
4. tap the loose end of the wire on pins 6-12, record results in excel.
5. move first wire end one pin up.
6. repeat steps 4 and 5 for pins 1-6.
If you're lucky enough to get the same keypad I used, you're work here is already done, and you can use the chart below to help in the next step
Step 6: Wire Button wires to Breakout Board
I'd definitely test each key after hooking it up to make sure you don't get all of them attached and have none of them work. Attach all your other buttons on the guitar to the breakout board, test it, and you should be getting characters typed when you press the buttons on your guitar.
Step 7: Cleanup/Reassemble the Guitar
Step 8: Download the Basic Game
Step 9: Configure the controls
Step 10: Adding Songs and Mods
Likewise, there are many Mods for FoF that allow for a different look to the game. These are generally found on FoF fan sites and forums. You can also create your own, though it may require knowledge of vector graphics editing, and software such as InkScape. Feel free to mess with the stock theme though and try your hand at homebrewing mods.