First, get a good mixer, like this:
Then get a couple of cables (1/8" to stereo RCA), like this:
and add this (RCA to 1/4" PL) to the RCA ends of one cable (2 pieces):
Connect the mixer outs to the soundcard line in and the soundcard line outs to the mixer tape in.
There are other more esoteric connections to be done for proper monitoring, but let's start with this connection first.