There are a lot of different ways to start a paranormal investigation, and you will need to figure out what works best for you. The most important thing is to help the family feel comfortable with you coming into their home. It is bad enough that spirits are violating their space, but now they have paranormal investigators invading their space as well.
Remember, you are the expert in this area, and you need to act with a high level of professionalism. You can’t come into their house acting like you have never seen a ghost before. Maybe you haven’t but you still need to use the scientific method to identify any possible earthly causes for the phenomenon. It would go a long way to know, or have some working knowledge for physics, force balances, fluid flow, and dynamics. That way you can really identify, or rule out potential physical world sources for the phenomena.
There is an emotional side to any haunting, and it is important to treat the family with a high level of respect and dignity. They have allowed you into their home and they probably have some theories about what is going on. You need to be receptive, but keep an open mind. I would listen to the family to figure out where most of the activity is coming from, and at what times during the day/night. What exactly they are experiencing and how it is manifesting. It is important to pay close attention, since this will help you quickly identify the hot spots. There may be other locations that look great, but you need to stick to the hot spots whenever possible.
You will need to ask the family when the activity started. Ask them have they been performing any types of occult ceremonies, witchcraft, voodoo, seances, or using a Ouija board. Any of these can be the source of the haunting, and it is important to keep in mind, that many people will not be truthful about what they have done out of shame. If you can, try to spring the question on them in a way that will allow them to save face as best you can.
If someone in the family has been practicing witchcraft, or dabbling in the occult there is a chance that the haunting could be demonic in nature. It is important to realize that just because there is the potential, it doesn’t mean that is what is going on. Let the family give you an idea of whether or not the haunting is demonic, and if so, how bad. Keep in mind that if there is a demonic presence, it can, and may follow you home, so make sure that you are spiritually and emotionally prepared to handle that situation.
Fortunately most of the phenomena tends to be caused by earth bound spirits, and elementals. Earth bound spirits are the most straight forward of ghosts, are usually what you think of when you think of ghosts. When someone loses their life in a sudden or unexpected way, or decides to take their own life, they can end up stuck in the astral plane. Basically a world that overlaps ours at times. Many times these earth bound spirits don’t notice the living any more than the living notices them, but sometimes the earth bound spirits spend all of their time watching the living, it all really depends on the spirit.
Elementals really are the most difficult, and least rewarding types of haunting to diagnose. Usually the existence of elementals don’t fit well with the family’s, or really many others belief system. They are often times created by a person intently focusing their energy (positive or negative), on something or someone specific. These entities tend to be created by a human, and can take on a life of their own, sometimes for centuries. This newly created entity is what is commonly referred to as a poltergeist, and they exist to carry out the intentions of their creator. Most of the time their creator doesn’t even realize what they have created with their angst. This is why it is important to keep your thoughts right, and always think positively whenever possible.
Investigations can and will lead you in some odd places, so you need to be flexible. Know what resources you have at your disposal, and use them. Look for past owners of the house, interview them if possible. Where does the property sit? Is it in a historic location? Can you find any historic property records? Is there any nearby cemetery? Did the house sit abandoned for sometime? All of these questions could provide clues during your investigation.
I would recommend planning to take some EVPs at every investigation. This activity doesn’t have to last for long, but it could provide you with some clues as to what is happening. Remember that not all EVP sessions are going to be successful, and that not getting any discernible EVPs doesn’t mean that the EVP is a waste of time. It is merely one avenue to collect clues. The higher the quality audio you are collecting will improve your chances of getting something useful.
Place as many high quality cameras as possible in the hot spots as well. The video is going to be your most compelling evidence, and it is important to get as much of it as possible. Capturing what is going on in a video will be a big vindication for the family, and it will make the whole investigation worth your while. Leave the cameras running over night in the hot spots if possible. I would recommend HD quality infrared video cameras. They need to have the highest video quality mode possible, and have the memory slots loaded so they can record as long as possible. Make sure they are well stabilized with a tripod, or something else, and that the zoom on them is as wide as possible on the hot spots.
Walking through the property carrying a video camera can be done, not highly recommended, but be careful with the camera. If you capture something, and the picture is bouncing all over, it will be worthless. You must always keep the end goal in mind, high quality video. Eliminate any factor you can that will reduce the quality of your evidence. I often see ghost videos, and they look like they were filmed with a camcorder from the 80s. Keep up to date with the technology, it has gotten much much better over the last 5 years.
When you are done, make sure you share with the family everything you found, and try to help them. Your not only ghost hunting for the fun of witnessing the unknown, but you are also trying to help a fellow soul in need. Give them useful steps that they can take to eliminate, or reduce the activity. Add reminders to your calendar to follow up with them, and make sure that they are following your instructions.