Welcome to the Forum DJ!
You’ll be wanting full-range measurements, which means you’ll need to get the calibrated Behringer ECM8000 mic and either a mini mixer with phantom power like the Behringer Xenyx 802, or a mic pre amp with a USB output. After that, it’s no different than using a regular stand-alone RTA: set up the mic and run the program. I’d smooth the readings for 1/3-octave. You can use REW’s RTA feature If you have an equalizer and want to be able to adjust it in real time.
Another option, you could use REW’s standard sweep feature and take readings from several locations and average them all. Then you could virtually tweak the response curve in the program, and then apply the filters to your equalizer.
Regards,
Wayne