When I’m working on a complicated subject in watercolor, I sketch it first on drawing paper. When I need to transfer the image onto watercolor paper, I simply use Scotch tape to adhere both papers, with the watercolor paper on top, onto my sliding glass door. The light shining through the glass illuminates the paper and the drawing underneath. I then trace the drawing neatly onto the watercolor paper. This works best on sunny days, but even when it’s overcast, I’ve experienced no difficulty transferring my drawing.