I stumbled upon this while prototyping a mixer a couple months ago. It's extremely simple to make and very very very few parts.
Parts needed:
(1) LM324N Quad Op Amp (though you only use one amp. You may be able to get away with using a single op amp for this, I haven't tried)
(1) 4.7K ohm resistor
(1) 10K ohm potentiometer
(2) 1/4" audio jacks
Instructions for breadboarding:
1. Place your LM324N IC on the breadboard, connect pin 3 (Input + 1) to ground (pin 11) with your 4.7K ohm resistor.
2. Add your input jack to pin 2 (input - 1) and ground.
3. For your output, connect one wire to ground going to one leg of the jack, and connect a 10K potentiometer on the other leg of the jack with one of the outer legs connected to the center terminal. The center terminal is connected to pin 1 (output 1) on the IC.
4. Connect positive power (9V) to pin 4 and negative to ground (pin 11).
Plug in your guitar, bass, drum machine, synth, etc. and play.
Let me know if you run into any problems with the walk through or the schematic (attached below). I'm writing this from school and don't have my finished circuit here with me, so it's possible I screwed it up.