Hypnosis isn't magic. It's essentially a procedure to bypass a person's conscious mind and deal directly with the subconsious.
Hypnosis is divided into two large groups: Stage Hypnosis, and therapeutic hypnosis.
Stage Hypnosis, uses large crowds and an interruption of an action or thought process to create a vulnerable moment were the consious mind is suspended. A command can be given, and the subject will be under a hypnotic trance.
Therapeutic Hypnosis uses calming techniques to relax the consious mind until only the subconsious is dealt with. Self hypnosis is a more difficult sub branch of this in which the hypnotist has to almost split their mind between an aspect that is ordering and one that is taking orders.
Hypnosis is a measurable phenomena. Using EEG measurements, scientists have found that hypnotized people go through a state that is similar to non-REM sleep.
The reason hypnosis seeks to access the subconsious mind is that all people have inherent desires and behaviors which are restrained by consious thought. A person not ruled by consious thought has no problem crowing like a rooster in front of a crowd because they would have no filter.
Interestingly enough, strong moral concepts cannot be bypassed by hypnosis, as things such as aversion to rape and murder are deeply ingrained in the psyche
Improving a persons life with hypnosis is based on post-hypnotic suggestion. In post-hypnotic suggestion, a person's subconsious is made to agree to a concept introduced underhypnosis, such as "Cigarettes are gross". From then on a persons basic nature would be changed slighty. Their concious mind would want to quit smoking, but their subconsious would help out by giving a feeling of disgust with cigarettes
Hypnosis itself is basic and readily doable. Post hypnotic suggestion requires a highly skilled hypnotist, who is capable of finding the right wording and behavior to make a permanent change. This is VERY hard.
Long story short, hypnosis is science not magic. The brain is a device that takes in stimuli and reacts and makes sense of it. Hypnosis provides stimuli or patterns of thought which temporarily change the way the brain works.
I've performed self-hypnosis many times and have hypnotized others, though I don't try to modify other people, because I'm not trained enough for that to be ethical.
I don't even recommend modifying yourself through hypnosis, because it can lead to unintended consequences and strange thought patterns.