Instructions
Grab four or five heavy books and set them out on a table.
Open one of the books to a random page. Leave the book open for now.
Uncurl your photograph as carefully as possible and insert it into the paper envelope. This protects the surface of the photo from any printing chemicals left in the book.
Insert the photograph into the book and close the book over the photograph, ensuring the photo lies flat within the pages.
Stack the rest of your books on top of the first book and leave the photo to sit for a couple of days. The photo will lie flat once you retrieve it from the book.
You can place several photographs in one book to flatten them as long as you put a few pages in between each one.