There is vast amounts of information in books and the internet explaining EVP's as well as actual recordings. As to the scientific part of EVP's I invite you to research this information on your own, as to how electronic recording differs from the way our ears work. Its all very interesting. Instead of filling pages with data I will simply provide you a list I have compiled that I use in the field. Everyone has their own methods of conducting investigations and I must say that EVP's are absolutely exciting and once you hear them you will be hooked.
1. Use good tapes, new tapes that are not reused. This eliminates bad tapes and eliminates reasons to discredit your recordings.
2. Use an external microphone if possible. This will keep the (recorder noise) off the playback. Keep the microphone ahead of you and away from your body.
3. Always record at the highest level possible.
4. If there are other members of your team present (or even if you are alone) talk in normal voices and never whisper. This helps alot during playback when you heard whispering voices on the tape.
5. As with photography, talk with the spirits. Again, talk in your normal voice and do not whisper. Ask them nicely to talk with you, ask them questions, be polite and always thank them.
6. During playback, play it at its highest level with headphones so you are able to hear everything in the tape. Although some EVP's are loud the majority are extremely slow spoken and whispers. If you are not listening close with headphones you WILL MISS EVP's. Sometimes the voices talk under your voice on the tape. You need to be patient and listen without interruptions
7. As with any investigation always stay positive and always expect ghosts to be there. Don't ask them if they are there, always assume they are.
8. Be aware of your surroundings, footsteps from your team, a furnace kicking on, a generator, rain on the rooftop, someone laughing. These will all come up on tape and its best that you know what they when heard.
9. Have an external editing program to listen to your tapes. There are many fine programs and I endorse none. Many times you will need to amplify the sounds, take out the hiss and the clicks, sometimes slow it down, and someplay you may want to play the tape in reverse.
10. If you do find something on the tapes, get different opinions, rule out possibilities. As with photography many people will discredit your work and offer excuses. Try to rule out anything and everything.
I guess recording is like taking pictures. It takes time and it is a required skill that you will improve in. Ghosts will talk only if they want to and if they are willing, so in saying so, always be kind, stay positive, keep an open mind, be polite and courteous and always thank them for speaking!