I've looked at a lot of stuff about REW, but most of it seems very hard to understand.

Just now I found a very good basic guide on getting a minidsp UMIK set up with REW, and how to get started measuring - in case anybody else might find it helpful:

UMIK-1/2 setup with REW (minidsp.com)

Unfortunately it stops there, and I haven't found a comparable guide on how to then use REW to generate corrective eq curves (and which curve would be the most accurate/neutral).

Since my set-up is old, simple 2-channel, I could set the corrections manually on the Behringer after that.

I found something from somebody who had a PhD in Physiological Acoustics that he had a hard time understanding and using REW - that made me feel a bit better about my trouble with that.