Download a free program like Audacity from the Internet or purchase a music recording program.
With a 1/4-inch cable, plug into the jack on your keyboard that says "Out/Mono." Connect the other end of the 1/4-inch cable to the 1/8-inch adapter. Plug this end into your computer's "Line In" or "Microphone" jack. Assign it as "Line In."
Open your recording program and turn on your keyboard. In your recording program, you will be able to have a mixer view. Go to that and set the input. It's usually located near the top of the I/O. Select the input that your computer uses for line in.
Go to the "Track View" in your recording program. Hit the "Record-Enable" button. This puts the track you will be recording on in record ready.
Once you are ready to record, use the "Transport" to hit record. Play your keyboard. You will see waveforms forming across the track as you play. When you are finished, hit "Stop." You can play back what you just played by moving the cursor to the beginning of the recording and hitting "Play."