Plug your USB interface into the USB port on your computer. Plug your MIDI controller into the USB interface. Make sure that the MIDI IN jack on your MIDI controller connects to the MIDI IN jack on your USB Interface and the MIDI OUT to the MIDI OUT. Some USB Interfaces have built-in MIDI controls. If this is the case, simply plug the device into your computers USB port.
Start Reaper. Once Reaper is loaded click "Insert" then "New Track." This will create a vertical bar at the bottom of the screen in the "Mixer" section.
Click the button on the bottom of the screen that says "in" on it. This will be on the panel that was just created by your new track. Select "MIDI Input," then "USB Audio Device." From there, select the Channel that your MIDI device is programmed for. By default this will be Channel 6.
Select "R" from the same panel. This will arm the MIDI track to record. When you are ready to record select the record button from the transport panel at the bottom. This will be a round button with a white dot in the middle.
Use your MIDI controller to play your music. Once you are finished select the "Stop" button on the transport panel. This will be a round button with a white square in the middle.
Double-click on the track you have recorded to open the MIDI editor. From here, you can edit the MIDI data you have recorded and precisely add or remove notes.