Remember, for the wax-resist technique to work, you must use a water-based paint, not oil paints!
Use a large brush that will hold a lot of paint, rather than a small brush that you have to keep reloading with paint. And rather mix up more watercolor paint than you think you'll need than run out! Then simply apply the paint boldly, across the whole card, and in an instant you've got a Christmas card.
Tip: If you've used a white wax crayon, paint with strong colors so the design stands out.
