What Anon Ymous said, but put them in sleeves first, like Printfile pages, then wrap the whole thing E-out around a paper towel core. Wrap that with a sheet of paper and tape or rubber bands.
How much it flattens depends on a lot of factors - the type of film/emulsion/base, the temp and humidity, etc. Also, they may not stay flat soon after removing, but if you keep in a book after removing, they might stay flat.
Try different things, like heat, but not too much.