It seems like it has taken me forever to get to this point but the under drawing is finally complete and I can proceed with the first step in the watercolor process, which is attaching it to the board.
I attach the drawing to a hollow core board. You can find them in any art store. I use either a smaller or larger board depending on the size of the drawing. For attaching I use a heavy duty stapler. Staple the drawing on the very outside edges of the paper. I will also occasionally tape the drawing to the board with either a wide artists or drafting tape, which will not damage the paper.
If you are using a 140 lb watercolor paper you must stretch the paper in water before you attach it to the board. I usually run it under the tap or submerge it in the sink with water for a few minutes. Take the paper out of the water and let it drip dry for a few minutes. It is important to attach the paper to the board while it is still wet so that when it dries it will be all stretched and ready to go!
The benefit of attaching the drawing to a wood board instead of your drafting surface is so that you can easily move your drawing around as you painting. Sometimes you will want to manipulate where the watercolor runs and being able to freely move the board will let you do this.