It's really up to you how you do it. Biologically speaking, you want to put them down at the same time for bed, and get them up at the same time in the am. They might make the change naturally, or they might need some help. I think a lot of moms are getting their lo's ready by putting them down 15 mins later for a few days, then doing it again, etc.
*HUGS*
Sonya =P