This will stop your paper cockling when wet, although some people don't mind a bit of undulation on the surface.
1. Leave your paper to soak for about 5 minutes
2. Meanwhile cut your gumstrip (it comes in thick 2 inch wide rolls generally) to size and find a board that wont bend easily (1 cm thick MDF or plywood should be fine)
3. Let the water run off and place your paper on the board. You can use a sponge to soak up surface water but I use an old T shirt.
4. Use a sponge and clean water to wet the gumstrip, but not too much (moisten would be the term).
5. Tape down the sides on to the board (about half on, half off)
6. Leave flat to dry. It is also fun to paint on the paper while it is still wet.
Alternatively you can soak the paper directly on the board with a spray or sponge (both sides) before taping it down, but I find this less reliable.
When you finish the painting you just use a craft knife to cut out the painting and reuse the board. If you want to get the gumstrip off your painting then re soak the paper in warm water and peel it off.