Get a simple two-RCA to a single 1/8th inch stereo cable. Connect it from the
"tape out" on your receiver to the "line in" on your audio card.

Take some time to optimize the bit rate of the capture. If you try for a really
high bit rate and your PC won't handle it the sounds gonna be crappy. Picking a
bit rate that's too law will also be suboptimal.

Also, note that ripping from LPs will have to be in real time.