I've "traveled" around religions trying to find my place. And used to lead a dark path, Not an evil path, but a dark one. I became obsessed with knowledge of theology, the occult, and especially demonology. I took it to heart when Sun Tsu said that to truly defeat your enemy, you have to know and understand that enemy. And the truth that is out there is definitely findable. I feel that I have opened doors no person should ever open. I've gone very deep into the occult and the things I saw, i cannot make them go away no matter how much I pray. It actually led me (back) to God. I feel a very direct connection. "His will, my hands" sensation. I have gotten good at all different types of cleansing houses and exorcisms. I feel I do it as my way of repentance. and I feel I am on the path He wants me to be. (I also feel I have been led to learn Orthodoxy, and I think my true place is with orthodox teachings the more and more I learn about them)
The point I'm getting at is demonic Possession is a very real thing. I have seen some terrible things (surprising for my age to be honest). but the main question was "How". I believe there are several avenues for it. a weakening of the mind. emotions like despair, depression, ect. invites things to your doorstep. but that falls under the unintended possessed. I have seen people wish for it, and sadly had received that wish. and it didn't require a crazy spell. demons just need an invite to take over.
We have free choice. we can choose to believe or not. allowing something into us to take over is a choice. but that doesn't mean God abandoned anyone. i think it does teach a humbleness to those victims.
I have spoken to a variety of demons inhabiting people. (there's little tricks to bring them forward for a direct conversation) and lo and behold! they are tricksters. They are full of half truths and manipulation. They love to watch you squirm, so surprisingly they will tell you alot of truth...it may be a little stretched, but with a good thought process one can figure out the core truths. They can't see the future per say, more that they look directly into your soul and tell you things to come on the path you are on. they know all of your sins, can read thoughts, languages, ect. they will throw truths at you to rattle your cage. and chip at your faith.
but that is a demon. a person being possessed will have a change in diet, experience pain towards certain metals or musical notes. know things that they shouldn't know. for example, they might tell you something about a childhood memory. Or mock you about a secret that you never let out before. all to shake you up. I've experienced thinking a question loudly in my head and had a verbal response from the other person.
scientifically, it is a remarkable thing to behold. spiritually, it is truly terrifying. nothing at all depicted in a movie. it is an impending doom sensation you receive. you feel your life in danger all the way to your core. and you have to allow God to flow through you to give you strength.