Page is signed a bit like paper, except you use your thumb for page, and not a flat hand like for paper.
To sign page, lay out your non-dominant hand, palm facing up, to act as the base or main book. Then take your dominant hand, form it into the ASL letter A sign with your thumb sticking out, and use that thumb to mimic turning a page on the flat hand by pushing it sideways against the 'book'.
Teach your toddler the page sign when you want them to practice turning the picture book or story book page on their own.
Teach your toddler this sign in conjunction with other signs pertaining to their picture or story book, along with the flash cards describing the actual story characters or objects seen in the story.