Adapted parts from Splitting a recording into separate tracks.
If using a record player, you'll need a phono preamp to increase the level from the record player to line level. Better still, use a record player that has RCA line out or USB out. The Audio-Technica LP-120XUSB has both. If the record player has USB out, you can connect that directly to your computer, and skip to the next section.
If using a tape deck, or a record player with RCA line out, you'll need to connect RCA cables to a DAC (digital-to-analog converter). For example, I use Livewire Essential Interconnect Dual Cable RCA Male to 1/4" TS Male, plugging the RCA Male to the "Line Out (Play)" jacks on my tape deck, and the 1/