This is a sample project from MAKE magazine, which shows you how to build a cracker box guitar amplifier for around $5 . It only shows the first few pages of the project, but it gives the schematic for the circuit and you should be able to work out how to finish it yourself. For the full article you will need to buy issue 9 of MAKE. Or why not subscribe: it's an excellent magazine for would be inventors and often features music related projects.

The design is based on an ampflier made by European guitar tinkerer Blind Lightnin' Pete, and was adapted for MAKE by Ed Vogel. You should be able to get all the parts and tools from your local electronics store like RadioShack (or Maplins in the UK).

If you decide to make it you will need the following bits and pieces: A box of some sort or another (a Ritz cracker box is shown), Toggle switch (single pole single throw), 9V battery, Battery connector, 0.047μF capacitor, 220μF capacitor, 0.01μf capacitor, 100μf capacitor, Hookup wire, 20 or 22, gauge AWG solid core is best, 5KΩ potentiometer (audio or log taper), 25-ohm (25Ω) rheostat, LM386N audio amplifier, 8-pin DIP IC socket, Chicken head knobs (2), Prototyping PC board, Soldering iron, Solder, Speaker, 8Ω impedance, 10Ω resistor, ¼" mono phone jack, speaker grill (optional) and Glue gun.

Link: Build a cracker box guitar amplifier (PDF file from MAKE Volume 04).