I suggest you just buy a mixer, it's easy enough to build a simple mixer with a single opamp, a few resistors, and a number of potentiometers - doubling it for stereo - but you can buy great quality mixers for very little money (check out Behringer ones), just the pots and knobs for your own would probably cost more. The bought mixer would provide a great many more facilites than a simple one you make your self - but if just line level inputs, and no tone controls, are all you require? - then it's not a hard job.