Install any desired drum VST plugins into Reaper's plugin folder. The default file path for this folder is "C:/Program Files/REAPER/Plugins/FX." Connect the MIDI keyboard to the computer, and the keyboard will automatically install any required drivers.
Open a project in Reaper. Click the "Insert" menu at the top of the screen and then click on "Virtual Instrument on New Track." Choose the desired drum VST from the list of available plugins that appears.
Arm the track for recording by pressing the "R" button next to the track name in the main window. Press the "Record" button on the transport toward the center of the screen. This will record any MIDI data sent by the keyboard to the drum plugin. Press the "Stop" button to end recording.