Discuss your decision. Talk to a trusted friend or family member about your thoughts about leaving the program. Examine your reasons for wanting to leave the Ph.D. program. Talk about or write down why you want to leave and identify your current priorities. Have your interests changed? Do you want or need to relocate to another area?
Make an in-person appointment with your adviser and attend the appointment.
Inform your adviser that when you applied for the Ph.D. program, your priorities were to finish and obtain a job in research (or insert your former priorities), but now your priorities have changed and continuing with the program is not in your best interest.
Elaborate on your priorities to help the adviser understand why you are leaving the program. Thank the adviser for mentoring and assisting you --- there might be a time when you need to use the adviser as a reference.