Take the painting out of a frame if it is in one.
Dampen the back of the painting with a wet sponge.
Lay the back of the painting flat on a clean towel and cover the surface of the front of the painting with newspaper. You should find a wide open space in your apartment or house. You can lay painting on a desk or a hardwood floor.
Cover the entire surface with hardcover books or a similar flat weight. Ideally, you would use a large marble slab, but assuming that you do not have a large marble slab in your apartment, hardcover books will work. You do not need to stack books on top of one another, only position enough books to completely cover the upward surface of the painting. Leave the books on the painting for at least five hours, or at least until the water dries.