How can you be possessed by the devil?

There are numerous ways in which this is possible.

If you wish to be possesed by the spirit of a human or a deity there are different ways of going about this.

Get to 'know' the spirit you want within you - think about it constantly, imagine what its nature is - what it's outlook on life would be, cultivate this knowledge within yourself.

Be aware of your thoughts and actions - stop every now and then, try and 'think' like the kind of spirit you wish to become possesed by would think. If someone speaks to you - say not what you would say, but what the spirit would say. This prepares your mind for the presence of the new energy and allows for easier integration.

Try this for about, say, 2 weeks, then include meditation. Enter into a deeply relaxed state (do this however you wish) and focus on something simple - like your breath or a sound in the distance, or the light of a candle. Prepare your surrounding environment to refect the nature of the spirit (if you're looking for an evil spirit just use decor that you would associate with evil) - this is one of the reasons the first phase is important - the more intimately you know the spirit and the more you think like the spirit - the easier this environment will be to create. Once you are relaxed, focus on the nature of the spirit - in this state do not think like the spirit, try not to think at all - just be aware of its nature, be aware of your nature, and observe how in the first phase - you made your natures as one.

It also helps to keep a dream journal - this encourages lucid states in dreaming where one can 'summon' the spirit for communication.

Possesion is rarely a sudden thing - it creeps slowly as you essence is aligned with that of the spirit. After a while, you should lose touch with you 'former' self - and be on a much darker path.