When I stretch my paper before painting, the finished painting doesn’t need to be flattened because it’s always flat. But if I didn’t stretch my paper beforehand then I would follow these steps. When the painting is finished and the paint is completely dry, I lay my painting, face down and dampen it gently with a little water using either a big brush or sponge. I then put another piece of paper (usually several sheets of copy paper or often a foamboard) on top of the back of the painting and place something heavy on top like books for example, and leave it to dry overnight. In the morning the painting is completely dry and also flat.