If I recall correctly, OCA has covered it on some of his videos on previous versions of his calibration tools, so I'd start looking there!
You will also find some of the settings you should use in REW on the Nexus Changelogs, though the sweeps you'll be able to use depend a lot on your system (windows or Mac + if you have height speakers or not).
From your comment, sounds like it's a Mac, so if you don't have height channels, you can do it with REWs own sweeps. Do keep in mind that sometimes when you connect the AVR to a mac through HDMI, you might have center / LFE channels incorrectly mapped (swapped), but I believe this can be remapped in rew.
I was about to test Neuron yesterday with my current Nexus measurements, though apparently the Mac version hasn't been released yet. I've read some users were able to run using Wine, though if you're not in a rush, it shouldn't be worth the trouble, as OCA has suggested that a Mac compatible version should come out in a couple of days.
There has been some teasing that the new tool that comes with Neuron might help the process of measuring, and apparently a new video on it should come out soon, so I'd also recommend waiting for it to come out (and hopefully Mac compatible by then).