Demon eating is a lost art form that was lost years ago. I have recontructed the art using metaphysics. It uses Focus of Mind, Affinity, and imagination. First things first, have a shield that will protect from demons. I will describe the shield protection needed. The shield is this: Think the creator keeps you safe and helps you out or think and you get what you want as he or she keeps you away from harm, this is used as you think about things so you do things correctly and without too much stress or frustration. This uses the subconscious of the creator of what you think to use, that makes it work and this is the main point that is there below the levels of perception.
The shield is needed as this also can be a focus of a gray wall around
yourself that deflects demonic attack and corruptive change, this is
the mental manipulation that is the demon who might attempt things for
you. The idea is to keep the demon back by fear and emotion, this is
only to keep away the demon that that cause revolt and to allow calmness
of mind then be away sometimes by thought to support it. This will power
the shield. This way keeps it working no matter what and even in weakness.
Now to strengthen it. Say "No I don't need this!" firmly to it mentally until the demon sudsides. This is the idea, used by thinking and that is what the shield represents. So you know that sometimes what the creator does. What else I noticed him or her doing, I think I saw him or her create a grey wall and that was to make by rubbing the hair. Used was a gray cloth to create the energy, that the aura was projecting by the energy of the spirit body.
Imagine while feeling very unbreakable much like unbeakable charged steel that you are stealing or absorbing the demons essence. Make a puckering face and suck inwards. This action will do an act of witchcraft, in that will enact a current of magic to draw the demons essence into you. When the demon is concentrated enough inside you. Say these words mentally "I wish the demons will be broken through its own energy." You can convert them as any demon too, by holding a little of them in you and saying convert or an erevert. That is how the witch does her deed.
Then the energy is yours to do as you want. However, it is still strong. So resist the evil things that the energy is going to do. When you trained it to respect you, it is truly yours to command. Say afterword, "I wish to become younger in appearance and keep my knowledge intact without becoming insane or stupid." or something similiar.
Thus the demon eating art is understood. But you do it without breaking or your condoled and controlled till you figure a way out of it. Demon energy is a force, that can live on its own. However, split up its weakened as a demon is a corrupt soul. So keep the body closed by wear of clothing and it won't get to you. Feel confident and feel that you won't be taken. That the demon will do as you command, then it will do as you say. As long as your focused. Use of slight tone and subtle suggestion is another way. At least it is a form that can be posed as a question. The idea is to be fair, so play fair, be nice and evil with the evil.