The Thermometer in Phasmophobia is probably one of the best tools for locating the Ghost. It can, however, be deceptive as it may be triggered when the ghost passes by it. Or the cold room may not turn out to be the room in which the ghost is the most active.
The Thermometer is probably the easiest tool to use. Simply have it turned on in your hand and walk around. Keep in mind though, temperatures can be deceptive. A passing ghost can trigger a momentary low temperature.
I've also seen many cases where a low temp room may not be the location of the ghost's most intense activity and it may not even mark the location to use smudge sticks. If you get low temperatures in a room and have trouble getting more evidence then you should search nearby rooms and try using your other items such as the ghost book.
In the Asylum and the High School in particular, I've seen cases where the hallway is cold, but there is a source room connected to that hallway that is freezing. The source room is the room in which the gear needs to go. I've even seen a case where all the downstairs and upstairs hallways in the High School were cold and we found a source room upstairs that was freezing. In that case it was cold in the hallways downstairs, colder in the hallways upstairs, and freezing in the source room.
There are even cases were more than one connected room will give Freezing Temperatures and I've also seen a few cases where eventually the entire floor of the location descends to freezing temps.
As a last note for using the Thermometer, you'll want to keep in mind that sometimes it can take a few minutes for a room to get cold. An example of this would be an entry hall ghost. You sweep the whole house with the Thermometer and the EMF and get nothing. Then you decide to head back to the truck to come up with a plan, and boom! As you are leaving you see your breath in the entry hall. So you may need to sweep areas more than once with the Thermo. Also keep in mind that not all ghosts will cause low temperatures, though in most cases the area should drop below 10C or 50F eventually.