what you will need:
Nintendo Candy tin (I found mine at costplus but I'm sure they're everywhere)
Electronics for a simple LM386 amp
(LM386 chip, 10uf and 250uf electrolytic capacitors, 10ohm resistor and .05uf cap)
a mono right angle 1/4th inch inch jack with screws (found mine at futurlec)
2-3 coin cell batteries (larger 3v ~CR2032 work ok)
small speaker (under 1.7inch diameter, i think i pulled mine from one of those talking birthday cards)
wire, soldering iron, solder, perf board, tape, dremmel and/or drill or nibblers, screws (for mounting the switch) etc...
Step 1: Step One: EAT CANDY!
(you don't have to eat it, you can put it in a baggie or somethig if you want)
you might want to clean out the inside of the tin to get rid of the candy dust in there.
make any measurements and realize you'll have to squeeze wire, perf board, electronics, a speaker, batteries and a switch all inside that little candies tin.
Step 2: Step Two: Electronics
note that this circuit is quite stripped down and only makes the guitar's signal an overdriven tone (distortion)
you could add your own version ex: like the little gem amp, but keep in mind you'd have to add potentiometers and there's not much space inside the tin for that
you can use an allegator clip to a guitar wire for testing rather than soldering to the jack before you put it in the tin
cut out a piece of perf board so that it will fit inside the tin ( allow some extra room for batteries)
solder it on to the perf board but add wire so you can connect it to the switch, jack, speaker and batteries
i've added a picture of how it might look on a breadboard and perf board
the yellow wire recieves the guitar signal
the green wire goes to one side of a speaker
the blue wire goes to the switch
the 9v clip is just to signify power in and ground
you can see that there is not much there, and you'll only need one or two hole spaces surrounding your LM386 chip when soldering it onto perf board (so about an 8x8 hole board)
make sure all ground connections are well soldered together, and you may need to have a wire touching the jack's ground (sheith) for a better sound.
Step 3: Step Three: Make Holes in the Tin
place the jack on the bottom and plan out how you will drill the holes
the main hole for the wire to the jack can be fairly large and should be around where the connector to the jack is
the other two holes are for the small screws for the jack
note that i'm only using the front side of the jack (it comes with a back but we won't need that)
it's a good idea to place the jack on there, then make marks through the holes where you want it and then drill
the tin is rather thin so try to be gentle when drilling
also, don't screw the jack in just yet, you've still got to solder it on and that's easier if you pull the wires through, solder it, then screw the jack on ( i would solder the jack and the switch at the same time)
again, solder it up before you really tighten it on there (the tin is thin so you don't want to keep putting it on and off)
also in this step, make a rectangular hole in the back side of the mushroom (away from it's eyes)
this is a little tricky, i used a nibbler but a dremmel may be better
try to get it so that you can get your switch in, but also have room to drill two holes on the sides so you can screw it in.
(note the picture)
Step 4: Finishing It Up
solder wires from the switch and ground for the batteries
i just had two button cell batteries, pressed the wires up against them, put tape around it and also wrapped a rubberband around it too
the batteries will last quite a long time if you don't forget to turn it off or practice with it on for hours
it's mainly meant to just allow you to play a distorted guitar anywhere without having to lug around an amp or even one of those tiny amps that still use a cord
Step 5: Test It Out!
make sure the volume knob on your guitar is up
it's not going to be incredibly loud but you'll notice your guitar will have that distorted rock sound
it fits right on your guitar with no cords, no clips or anything, just the jack
(you may want to hold the jack when unpluging it from your guitar to ease the force off of the little screws holding it on the tin)
note that this might not work for guitars with in-set jacks (like a stratocaster)
super secret: turn the amp on, press the tip up to a piece of metal, you might hear a radio station