Anyone who has ever painted with watercolors (either as a kid or adult) knows that the paper "buckles" when it gets wet. It makes the paint roll off of the high parts and settle in low parts. Making for a not so nice finished look. I'll show you how to solve this problem. Stretching your paper. I use Arches 140 #(pound) cold press paper. For beginners I'd recommend using a quarter sheet and work your way up to a full sheet. I fold and tear the paper to keep the deckled edge. I fold the (full sheet) in half one way and then fold it the other way and then tear it. You'll then have 2 half sheets. Then I take one of the half sheets and fold it in half one way and then the other way and then tear it. Leaving me with a quarter sheet.
Then place your desired sheet of paper in a bathtub with warm water for about 10-15 minutes. Turning it over about half way through.
When it's finished soaking take it out of the water and hold it up over the tub, letting the water drip off until the dripping stops. (About 10 or 15 seconds) Then place the paper on your board. HERE is where I describe what kinds of boards are suitable for Watercolor paper. I usually smooth it out, making sure it is flat on the board.
Then staple the paper onto the board. Keep the staples about a 1/4 inch from the edge and about 1 and half to 2 inches apart.
Your paper should look like this when it's done. (This is a half sheet, not a quarter sheet)
When it's dry, you can paint your picture, and when you are finished, use a a pocket knife (See the above link) to remove the staple by gently sliding it under the staple and the board and wiggling it side to side until it comes loose. Usually about 90% of the staple come out entirely but I sometimes have 3 or 4 that I have to pull the rest of the way out with a pair of needle nose pliers.