I'd do it in increments. If you normally get up at noon, try getting up at 11:45 AM. Put the alarm clock clear across the room so you have to get out of bed to turn it off. Once you are out of bed do not go back to bed. Then after you get up at 11:45 AM for a week, get up at 11:30 AM. Then do that for a week. Then 11:15 AM. Then 11 AM etc. until you get to the desired wake up time.