A written birth plan covers everything from whether to use pain medication, personal preferences during the delivery, people who may be present in the birthing room, any alternate birthing techniques, preferred birthing positions, and a written document for nurses and doctors so there is no miscommunication about your preferences.
Of course, childbirth is unpredictable and your birthing plan needs to be adjustable and make room for allowances. There are plenty of examples of birthing plans and documents that can be used as a starting place. Have a professional experienced in childbirth, such as a midwife or doula, look over the plan before making it final. When it is finished be certain your doctor has a copy to be placed in your medical chart. Keep a copy in your hospital bag to take with you in case you end up delivering in an unexpected hospital.