The EMF Reader in Phasmophobia is one of the basic tools. One of these EMF Readers will be in the truck by default when you start an assignment in Phasmophobia. You can purchase additional EMF Readers and bring them along in the truck, so that multiple players can sweep the building at the same time.
Using the EMF Reader in Phasmophobia is as simple as selecting it in your hand and turning it on. As you make your way through the location it may give you variable readings. If your objective is simply to detect eveidence of the paranormal with an EMF reader then all you need is a reading of two or higher on the EMF Reader and you'll have the objective.
But when it comes to Evidence in your Phasmophobia Journal only EMF Level 5 counts. Lower EMF levels can help you locate the ghost, but you need EMF Level 5 for it to count in your journal towards ghost identification. An EMF Reader can also be placed on a surface or on the floor itself and will be triggered if a ghost comes close to it.
Once you find the room, a common technique is to put two EMF Readers in the room, so that you don't have to keep them in your hands. If one of them goes off you'll want to get a good look at it before the event stops. If you pick up an EMF Reader while it is going off, it will possibly reset and you could miss the reading that triggered it. You should place it in such a way that you can clearly read it without having to pick it up.
Once you have either 3 pieces of evidence or an EMF Level 5, the EMF Reader has served its purpose. After that it is only a noisy distraction for your team and the ghost, so your best bet is to simply turn it off.