Connect your turntable to your stereo. Using an RCA cable, plug one end into the audio output on your turntable and then plug the other end into the phone inputs on your preamp. Then, using another RCA cable, plug one end into the audio output on your preamp and the other end into the audio input on your stereo.
Connect your stereo to your computer. Using an RCA to 1/8-inch cable, plug the RCA side into the audio output on your stereo and then plug the 1/8-inch side into your computer's line input.
Record an LP into your iMac. Open a new project in an audio editing program, such as Garageband or Audacity. Then, press "Record" in your audio editing program. Next, press "Play" on your turntable. Then, let the audio editing program record your LP. Once the LP has finished playing, stop the recording. You can then export the recording out of the program and into your media player.