Here's some boring technical information. To paint on watercolour paper that is less than, say, 400 gsm, it is best practice to stretch the paper before painting on it. This is so that the paper will not buckle when you paint on it - a smooth surface is often preferred for that professional look. Stretching paper involves soaking it in water, then laying it across a board, draining off excess water, and taping it down with specialised gummed tape. As the paper dries, it shrinks - and this is the tricky bit. You don't want the shrinking paper to tug your gummed tape off the board, otherwise the paper will dry unevenly. You want everything flat, flat, FLAT but so many factors can come between you and your goal of flatness.
Stretching watercolour paper the traditional way (with gummed tape) is a hit-and-miss affair. I get very stressed out with this part of the painting process. My latest effort was looking to be a disaster, with the paper buckling even before I finished taping it down! I managed to smooth it out again, but a few hours later, sure enough, the gummed tape was giving way.
This is the solution so far (and it looks like it's working!)... Stacks of heavy books (thank you, Complete Works of Shakespeare!), weights and G-clamps: