If using EZ Drummer, first thing would be to set up multiple outs and use the 'route out' setting on the mixer. In reaper, you sent up an aux channel with a compressor set to a very high level, for example -30db with a high ratio. You then send the parts you want to that Aux channel, usual thing I do is just send Kick, Snare and Toms but there's room to experiment of course. keep the fader right down and then slowly turn it up and mix to taste.
This is a very very basic outline as I am not much more than a bedroom warrior tbh and it would be worth doing your own research to get a better and deeper understanding. There is an article in Sound on Sound this month all about parallel compression so check it out and the internet, hell there is even a wikipedia entry on it.
Oh and put a high and low pass filter after the compressor on the AUX channel