To access any LifePics services, you have to first create an account if you don't have one, or login if you do. You can do both from within WPV by tapping the LifePics cell in the Libraries section, or by choosing LifePics Login from the Login menu.
After logging in, you can upload to LifePics any photos that are in the Photo Library section: Photos in the Photo Library, photos in the WPV Library (FTPed to WPV), or photos in your Dropbox. You can also upload any photos in saved albums on the QuickList.
When you upload to LifePics, you're given an option to go directly to the Order Prints screen.
You can't directly upload to LifePics anything on Flickr, Facebook, or SmugMug, but you can first download them, which adds them to the QuickList, and then upload them from there.
You can print anything uploaded to LifePics (from within WPV or any other way). Choose Order Prints from the Grid View action menu. You can also choose View Cart from that menu.
You can also choose Stores or View Cart from the home screen LifePics Album list.