Become a ROCK GOD for free.
Sorry I didn't think of this as an instructable until project was finished. But I think you get the jist of it all. First do some curb shopping to find an old portable boom box. The one I found was perfect. Radio still worked, tape player did not because it had a broken belt ( who listens to cassettes anymore anyway). The only thing you need to buy, unless you can salvage one from an old stereo is a 1/4 inch stereo phone jack (Radio Shack # 274-0312).
After opening boom box look for the audio cable running to the tape head and snip it at the tape head. You should see 3 wires on the audio cable red (right channel) white (left channel) and black for ground. You will likely have to solder some wire to these to get the length needed to reach the mounting point of the jack. I chose to mount my jack on the side where there was plenty of space.
After your jack is mounted to the boombox, you will need to solder the audio cable to it. The ground goes to the tip connector terminal, the right and left channels go to the other two terminals of the jack, which terminal gets red or white does not matter. Once audio cable is soldered to jack, you will need to snip the power wire to the cassette drive motor and insulate it. This is not really necessary if you don't mind the motor running all the time you are jamming on guitar. I chose to snip mine. Make sure all wires are insulated, if you have cd player, keep wires out of that area.
Plug in guitar cable, select cassette, hit play, and jam away.
My guitar has some sensitive pickups. Real easy to get lots of heavy metal sound through the boombox.
Guitar pickups work similar to cassette head, both turn magnetic induction into small electric audio signal. Boombox amplifies guitar just like a cassette tape. So ROCK ON MAN!!!