Hypnosis has three defining features: absorption, dissociation and suggestibility, any of which can be experienced apart from a hypnotic state; hypnosis can be said to be the concurrence of these elements. Heres how these three features are described in the Journal of Physiology-Paris: “Absorption is the tendency to become fully involved in a perceptual, imaginative or ideational experience. Subjects prone to this type of cognition are more highly hypnotizable than others who never fully engage in such experience (Hilgard et al., 1963 Hilgard, E.R., Lauer, L.W., Morgan, A.H., 1963. Manual for Standard Profile Scales of Hypnotic Susceptibility, Forms I and II. Palo Alto, Consulting Psychologists Press.Hilgard et al., 1963). Dissociation is the mental separation of components of behavior that would ordinarily be processed together (e.g., the dream-like state of being both actor and observer when re-experiencing autobiographical memories). This may also involve a sense of involuntariness in motor functions or discontinuities in the sensations of one part of the body compared with another. Suggestibility leads to an enhanced tendency to comply with hypnotic instructions. This represents not a loss of will but rather a suspension of critical judgment because of the intense absorption of the hypnotic state”.
Hypnosis is typically induced by following a sequence of steps, sometimes called the “induction script” (examples here). These will first lead the subject into progressively deeper states of relaxation, aiming for a total release of tension. The subject will then be lead to focus full attention on some object. When absorption in the object of focus is achieved, the suggestion is given.
The particulars of what cues and visualizations are used vary tremendously from one practitioner to the next, and some will be far more effective for certain subjects than for others, depending on their basic temperament. Some people go easily into a deep trance, while others are virtually impossible to hypnotize.
Self-hypnosis is perfectly feasible once the process is understood. The recommended way to learn is to first undergo hypnosis by a bona fide hypnotherapist, so that you can find out how susceptible you are to hypnosis and can experience what descent into a trance feels like. The therapist can even give suggestions that will make it easier for you to self-induce a trance.
Then it just becomes a matter of persistent practice. It is said to take months of regular practice to reliably self-induce, but people who become proficient at it can enter a trance almost instantly.
In case you’re wondering, I’ve done lots of reading on hypnosis because I’m interested in various aspects of consciousness, but I’ve never experienced hypnosis myself (and no, I’m not interested in hypnotizing anyone)