One of the great features built in to the Behringer XR18 Digital Mixer is that you can feed up to in the signal processing chain. This allows you to record individual raw (or processed) signals from each channel strip and do a proper mix later.

The recording process doesn't take a high powered computer either. I've been using an older, relatively slow laptop to record 16 tracks plus L/R at 48K 16Bit into a Reaper DAW with no issues.

If you're using a Mac you don't need any drivers for Reaper, but on a Windows machine, you'll need to install the ASIO4ALL driver that you can download from HERE

Here's a YouTube video that explains the setup process. What he left out is that after you've set up your XR18 and Reaper, you might want to save the XR18 setup as a scene preset and the Reaper setup as a project template so that you don't have to go through them manually each time.