Links to online articles and tutorials
This is a good article on stretching watercolor paper. Four methods are described: gummed tape, staples, wheat paste, and stretching on wooden stretcher bars.
You will need one or more plywood boards bigger than the paper. MDF would work equally well and might be cheaper.
In all cases the painting is cut out after stretching and the remains of the tape/paper are removed afterwards. This highlights the fact that stretching watercolour paper is a messy business. Is it worth it? Each person has to make up their own mind on that.
Blocks of watercolour paper are good too, with the paper attached around the edges so that it dries flat. Leave the painting on the block until it's dry, then remove carefully with the aid of a flat blunt tool such as a table knife, easing it off rather than cutting. (See sample Arches block at right).
An excellent tutorial, plenty of detail
Gary Reef's website