1. Wet the paper, then flatten it, preferably sandwiched between two books, press it with a heavy object, and put it in the freezer of the refrigerator. Take it out every other day, and the paper will be flat. This is a better method. Sometimes when a book falls into the water, it will wrinkle when it dries. You can also use this method.

2. Press the paper with a wet towel and iron it.