When you buy a poster it usually has been curled up tightly into a cylindrical shape. Before hanging it, the curl needs to be flattened out to make it easier to hang on a wall. Uncurling paper, whether a photograph, poster or a large written document, can be done with a professional tool or with a few household items. Be gentle with the paper so that wrinkles and tears don't form when you are working with it.
Invest in a D-roller if you have to unroll large prints often. This device can be found online and at some speciality photography shops.
Unroll the D-roller. Place the curled paper so that the curl faces down onto the flat part of the D-roller. It should be up against the cylinder part of the roller.
Roll the cylinder over the paper until you have rolled up the entire piece of paper within the cylinder. Hold it a few seconds and roll the cylinder back to unroll the paper. If the paper is still curly, repeat the process. The more curled the edges are, the longer you want to hold it within the D-roller.
Unroll your paper so that it lays as flat as possible, with the curled edges facing down onto a clean, flat surface.
Place a heavy book gently on each corner of the paper. If there is still curl along the edges, place books on these as well.
Leave the paper under the books overnight or for about eight hours.
Remove the books and you should have a flat piece of paper. If you don't, put books back onto the piece of paper and wait another eight hours.