There are three commonly accepted classes of voices that can be attributed to an EVP and here is a very general explanation of what they are:
Class A – A voice or sound that can be heard and understood when played back over a speaker.
Class B – A voice or sound not clearly heard over a speaker and could be debated as to what it says.
Class C – A voice or sound only heard through headphones and even then maybe difficult to understand.
Before you start talking to the spirits, you should place your recorder on a flat surface where it won't be disturbed. You should also state into the recorder what room you are in and the time and date, this will come in handy later during review.
It is always a good idea to introduce yourself.
Start out by asking general questions. Is there anyone here? Can you let us know you're here? Wait 15-20 seconds in between each question to allow time for the spirit to respond.
If there is a name associated with the haunting, try calling the spirit by name. Similarly, if there you know anything about the spirit, such as who their family members are or significant moments in their life, bring them up. Ghosts are more likely to respond to things they are familiar with.
An EVP session can be as long or as short as you want it to be. Once you're confident you've asked everything you wanted to, say your goodbyes and thank the spirit for talking to you.
Once the investigation is over, transfer all your audio files to your computer and a secondary device, such as an external hard drive, if possible. It's always good to have back-ups in case something happens! Listen to your audio closely. Listen for voices, footsteps, and unfamiliar noises. If you do hear something, think of the alternatives of what it could be. Let someone else listen and get their opinion. You can download free software, such as Audacity to cut down your clips to show to your team members and clients. I recommend making hard copies of all EVP's found. If you ever have to go back to a location, it will be helpful to look at the past evidence.
Make a note of any audible sounds while recording. For example note a cough, walking, car going by outside window or anything to help tag the audio with an explanation of what someone listening later might deem unusual. Try not to whisper during a session as well, if you are going to talk, say it out loud so there is no confusion later as to where the voice came from.