This is a just a basic 2:1 mixer I built a little while ago. I needed something to blend clean and distortion sounds in my guitar rig, and this fit the bill nicely.
As you can see, it's very basic. Such simplicity is not without limitations, so before you run out and build this thing, let's take a closer look at it. First off, the circuit assumes both sources have roughly the same output impedances, and that those impedances are small with respect to the 10k input potentiometers. If you plug a guitar directly into this mixer, it will sound quiet and very dull. Second, the DC blocking caps on the input are after the potentiometers. I did this for no particularly good reson other than it made the circuit slight easier to build. It would be best that whatever is connected to the input have no DC offset, as potentiometers will typically go intermittent sooner when they are controlling a DC voltage (though they should still last a good long time, so it's probably nothing to worry about). Finally, there is about a 6dB insertion loss on each channel (but the output is unity when both channels are full up).
I find it very useful to draw up some kind of basic layout before starting a perfboard circuit. This is close to how my circuit went together, though I did make a couple small changes along the way.
From left: