You need to have the microphone calibrated at a place such as
WCCL, or alternately (and less accurate) you need to borrow a cal file from someone who has had their microphone calibrated. Then you just make a file that can be read by REW.
The problem with borrowing a cal file is that you don't know if the mic you own is consistent in its response between the same model. We found the ECM8000 and the Galaxy is fairly close and offer the cal files on our download page. It's not as accurate as a real calibration, but certainly better than not using a file.
Most microphones won't measure flat down to 15Hz as required for subwoofer measuring, and so usually a cal file is needed.
brucek