This is a really easy technique for making painted cards that look very professional! It is also my new favorite technique for combining digital design and painting: using the Silhouette digital paper cutter to cut vinyl stickers as paint masks on watercolor paper.
The results are extremely crisp. There is absolutely zero bleeding, even if you use a ton of water. It really surprised me how well the vinyl keeps out the watercolor.
The main tool here is a digital paper cutter. I've got the Silhouette Portrait, but there are many, many others. I like the Silhouette because their software is very easy to use, and they have a pretty extensive design store.
I used the Silhouette design app to create a few different stencils to try (screenshot above). I tiled several of each and selected "Silhouette Vinyl" as the material.