(Content based on Mind website)
Before you can be detained under the Act, you must be examined by two doctors and interviewed by an Approved Mental Health Professional or AMHP (usually a social worker, nurse, occupational therapist or psychologist). Preferably you will be seen by these professionals at the same time, but sometimes circumstances mean that they will have to see you at different times. In all cases the two doctors will decide if they agree that you meet the criteria for detention under the Mental Health Act 1983 and, if satisfied, complete recommendations to this effect. The AMHP must interview you and be satisfied that detention in hospital is, given all the circumstances of your case, the most appropriate and least restrictive way of providing the care and medical treatment you need. If the AMHP agrees then the AMHP will make an application for you to be detained against your will at a hospital.
Upon completing this application the power is given to the AMHP (or someone nominated by the AMHP) to take you to the hospital that has agreed to accept you as a detained patient. When you are at the hospital, you will then be admitted onto a ward and the papers provided by the AMHP gives the hospital the power to detain you against your will.
You can find out much more information about how assessments are carried out under the Mental Health Act on the following websites: