Step 1: What you will need
1. A cassette player with a speaker (Preferably also with an equalizer so you can change the sound settings just like on a real amp).
2. A regular instrument cable.
3. A wire cutter/stripper ( I used a fruit cutter knife :P)
4. A Solderer.
5. Rosin-Core Solder.
Step 2: Instructions
-Open up the cassette player. What you have to do now is to take the two wires from the stripped instrument cable and try to locate the wires in the circuit that gives the sound to the speaker. For me, these two wires looked identical to the wires in the stripped instrument cable, but smaller. To know which the right ones are, simply plug the remaining plug into the guitar and take the instrument wires and find it using trial and error.
-Take the solderer and heat it up. Now take the solder and place it at the spot where the wires of both cables touch (which is 2 spots). Carefully press the solderer onto the solder and it should instantly melt the solder which glues the wires together.
-Make sure everything is dry and find a spot where the instrument cable can leave the inside of the cassette player (I removed one of the buttons and just placed the cable there). Now it's time to put back the cover of the player and reassemble it.