Step 2: Assembly
If you only use one speaker be sure to combine left and right audio wires so as to get complete sound.
If you choose to include an amp, try ripping one out of an old stereo or radio. Chances are you will be able to find one at a garage sale or Craig's list. This amplifier http://www.instructables.com/id/Simple-Powered-Pocket-Amplifier/ may work quite well.
Be sure that you make the wires long enough so that you can move your mp3 player and speaker around to fit your backpack.