Find ShortCircuit from a reputable source (KVR will probably have a link), put the DLL file in your VST plugins folder. Open Reaper, create track, open FX, click 'add', find SC. You should be able to see SC's keyboard. Open the folder your samples are in. Drag samples one by one onto different keys, SC should do the rest. That's a simple way to do it. Have a play and get back to us.