Here's a very basic explanation. Apologies if it's too basic, but it seems like this process is quite new to you. Your keyboard outputs two things - audio and midi. The audio is the actual sound that you hear coming out of the speakers on the keyboard (or alternatively, through the headphone jack). The second is midi, which contains no sound - just a set of data that stores which keys you press, how hard you press them, how long you hold them down, etc. The midi data is sent out through the USB connection. So you have two options: 1. Record the audio signal. You need a cable that goes from your headphone jack to an input on your audio interface or soundcard. Connections will depend on your system. Put a new track in Reaper and set it to record the audio from your interface. Advantage? Very simple. Disadvantage? One it's recorded you can't really edit it, other than dubbing corrections over what you've already recorded. 2. Record the midi signal. You do this by creating a new track in Reaper and setting it up to record the midi signal that comes in through your interface. As explained, midi has no sound, so you have to put in some sort of synth or virtual instrument. This goes in the FX slot for that track. Advantage? You can edit the midi data - change a note, add a note, change velocity, etc. You can also change the voice to whatever you want by using different VSTi synths or instruments. Disadvantage? None really, other than you're using a voice from a VSTi rather than those contained within your keyboard. If you're recording midi, you have to "record arm" the track and you have to "monitor input" on the track so that you can hear the virtual instrument while you're playing. This all sounds a bit complicated, but it's quite simple. Once you set it up once it'll be a snap next time.