According to the Encyclopedia of Britannica hypnosis means:

hypnosis, special psychological state with certain physiological attributes, resembling sleep only superficially and marked by a functioning of the individual at a level of awareness other than the ordinary conscious state. This state is characterized by a degree of increased receptiveness and responsiveness in which inner experiential perceptions are given as much significance as is generally given only to external reality

I have never heard of it being forbidden, but I have also never heard of it being allowed either. To me, it looks like its affects resembles to two different haraam things,

• Alcohol: As alcohol, it affects your natural way of thinking. It alternate the way of your thinking superficially.
• Music: I don't remember the exact reference, but I remember it is being argued that one can get influenced greatly with the message delivered by music without giving much thought about it, or it might evoke superficial emotional state. it resembles the way how delivering a message is easy with hypnosis and how it might evoke superficial emotional state.

Considering this, how should a Muslim approach the practice of hypnosis?

Edit: I am not talking about self-hypnosis, which resembles more to meditation, since there can be many differences in rulings between hypnosis made by one person to the other and self-hypnosis. This question is focused about hypnosis made by one person and affecting the other.