How do you get a hallucination?

Hallucinations: Elevated levels of Dopamine in the brain, such as due to psychiatric disorders, substance abuse, medications side-effects, sensory loss (vision, hearing) , sleep deprivation, severe fatigue, seizures, delirium, advanced stages of dementia

Hypothesis : One theory that could apply to your question is what's known as the dopamine hypothesis.Dopamine is a nerotransmitter which, according to the theory, is in excess. That means there's too much dopamine in the neuroreceptors, and this contributes to hallucinations.