It's generally recommended that watercolor paper less than 356 gsm (260 lb) is stretched before use, otherwise, it'll warp. It's a simple process.
Time Required: On-going
Cut four strips of gummed brown tape, one for each side (edge) of the piece of watercolor paper. Cut these slightly longer than the sides. Put them aside for the moment.
Soak a sheet of watercolor paper in cold water for a couple of minutes. This is to allow the fibers in the paper to expand.
Lift the sheet of watercolor paper, and gently shake off the excess water. Place it on a drawing board, which must be lying flat.
Smooth the watercolor paper out with a clean sponge (preferably) or your fingers (but wash them first to get any grease off them). If the sheet of watercolor paper is not perfectly smooth at this stage, it won't dry smooth.
Moisten a strip of gummed tape, and stick it down firmly along one side so that one-third of the tape is on the paper and two-thirds on the board. This will stop the watercolor paper pulling off the board when it dries.
Tape down the other sides of the sheet of watercolor paper in the same way.
Leave to dry for several hours, away from direct heat. As the water evaporates, the fibers in the paper contract, leaving the sheet of watercolor paper flat.
Keep the board flat while the watercolor paper dries, otherwise, the water will drain to one edge and the paper will drive unevenly.
When you paint on the watercolor paper, it'll stay flat because you won't ever soak the whole piece as much as you did in step one.
Don't use hot water to soak your watercolor paper as this could remove the sizing from the paper, and don't soak it for too long for the same reason. Sizing is added to watercolor paper to reduce its absorbency.
Use different color sponges for smoothing a piece of paper and moistening gummed tape, so you never run the risk of getting gum on your sheet of watercolor paper.
If you don't get on with gummed brown tape, an alternative method is to staple the paper down onto the board instead.
You may be able to peel off some of the tape, but be careful not to tear the paper. Rather, just trim off the edges of the paper or hide them under a mount.