Reaper - General Help

[MetalRenard]

I'm aware that you can make your own 'beats' in Reaper somehow (I haven't gotten around to that)

Midi editing ftw. Watch my video tutorial - this section is about drums specifically, but the same technique applies to all other midi instrument you use.
EDIT: Fucking Media embedding software, I can't post a timed link to a youtube video without it showing the actual video and not the link.
Jump to exactly 8:30 into the video for the bit about drums and midi editing.
http://www.youtube.com/watch?feature=pla...xf8#t=511s
Sorry about the bad audio quality, it was my first tutorial and I was still working out the software.

I've made a couple backing tracks in another software, some drum and bass and such, which I would like to record over, how do I do that? I want to take my FL tracks, render them to mp3 or wav or whatever, and jam over them.

You just gave your own solution to your question buddy. :)
Export as a .wav file then drag and drop onto reaper.