The online pre-order starts at 12:00 AM PST (that's 3:00 AM if you live on the East Coast so drink lots of coffee if you're not used to staying up late
). This is early,early Friday morning (January 10).
My advice is to check the Disney limited Edition Dolls Facebook page for the links to the Disney sale pages rather than just refreshing Disney.com. The reason is that someone usually posts those links in a couple of minutes after the start of the sale, while the Disney site can take as long as 15 minutes to add them. This really matters for the dolls that sell out in minutes. Be sure to be logged in at Disney.com, and it's not a bad idea to have your credit card info already stored there if that's how you will pay.
Pre-orders are also offered at Disney stores, but not all of them, so you need to call to see if your store will have them. They go on sale at store opening and are first come, first serve, so you need to line up early. I have never pre-ordered in a store so I can't tell you how early. The CM at your store can hopefully give you pointers on that. The store pre-orders occur after the online, so you at least have another chance.
Charges made by credit card at stores are immediate, meaning you pay in full at that time. Online you are charged at time of shipment, but your credit card bank may choose to put a permanent hold on your card until that time. Bill Me Later puts a temporary hold and then releases it until Disney puts the charge through again when ready to ship. I strongly recommend you NOT use a bank debit card online since it's possible to get to the point where you give Disney your card info and they will send it through for pre-approval even if the doll is no longer in stock. This means a 10-day hold will be placed on your bank funds even though you were unable to buy the doll(s). I know this from personal experience. Using a credit card with a large balance is preferred.