The humidification and flattening technique is the process of keeping paper in an enclosed area and moistening it by introducing a water source. This technique is useful for flattening drawings that have warped with time, as water vapor helps the paper fibers relax.
As a result, you can open the document safely, press it, and let it dry, so it becomes flat. It's also possible to create a homemade humidification chamber with a plastic container, towel, water, and light diffuser panel.
For flattening the sheet, you will need several blotters, glass, and weights. But if the sheets have adhesives, use spun-bonded polyester to prevent sticking.