Here are some illustrations I’m working on for a project. The process involves first doing a pencil drawing of the shape, which I then scan and trace in Adobe Illustrator so I have a clean-lined vector shape. Then I print it out as just a black outline, and tape to the back of a piece of watercolor paper.
Next, I place the sandwiched sheets of paper on a lightbox, and paint with watercolor. Finally, I scan the painted pieces and use the vector shape as a clipping mask to regain the crisp edges of the original drawing.
It sounds like a long process, but it’s a wonderful way to combine the organic flow of the watercolor with the sharp lines of a vector illustration.