Using a dedicated measurement microphone is the safest bet, and there are several really affordable options available these days. Preferably pick one that comes with a calibration file. This means they have measured your specific microphone and put the deviations from flat (in dB) in a simple text file. Import this file in REW and it will compensate for the mic's deviations so it will function as a microphone with a completely flat frequency response. If you want to do full-range measurements this is a must-have, but if you plan on doing low-end only measurements, eg you only want to integrate your sub with your monitors, a calibration file is not essential.

Preferably pick a regular XLR connected version, not a USB one. Although the USB ones sometimes have the added advantage of having their (SPL) level calibrated, for the type of measurements we need to do this is not necessary. But we do need to be able to connect the mic directly to the same interface your monitors are connected to.

Measurement microphones are always small-diaphragm condensers/electrets. They have an omnidirectional polar response, which means they are equally sensitive to sound coming from all directions. They will still get more and more directional from approximately 2-3khz or so, depending on the exact size of the membrane & housing.

Most measurement mics are designed for 'free field' use, which means they should be aimed directly at the source you want to measure. Some are calibrated for 'diffuse field' use, these should be angled (usually 60 or 90 degrees) towards the sound source.

Reflections from the mount + stand can also cause trouble if you're measuring high-frequencies, so try mounting it with as little as possible reflective materials.

If you're only interested in measuring low-frequencies (say below 300 Hz) it all becomes slightly less critical.

You will still need an omnidirectional microphone, but any omni you happen to have lying around will probably be suitable. It has to be an omni, and not eg a cardioid or figure-eight because omnidirectional microphones don't suffer from the so-called proximity effect. This proximity effect causes an (in this use case undesirable) boost in the low frequencies. Because you're dealing with relatively large wavelengths when you're only measuring low-end you don't have to worry about the size of the microphone/capsule.

If you don't use a measurement mic but a regular omni you have lying around, the mic probably won't come with a calibration file and you won't know how linear it is. Most of them are fairly flat though in this region and most of the deviations within individual versions of the same brand/type seem to occur in the mid to high frequencies. Of course, you can always try to find a reliable test/measurement of the mic you would want to use to find out if the brand/type in general is somewhat linear (=/- 2dB or so) in the range from 20hz up to 1000Hz. This upper limit is useful for determining the target level the lower frequencies should be matched to (with a calibrated measurement microphone an even higher frequency range is used for this purpose).