One of the easiest ways to create your own, handmade Christmas cards is to use watercolor paints and wax resist. It's also a technique that can be learned and used very quickly, so it's ideal if you've left making your Christmas cards to the last minute.
How does it work? It's based on the fact that oil or wax and water don't mix, so you draw with the wax on a sheet of paper or card, then paint over it with a water-based paint. Where the wax is, the water in the paint is repelled. The paint either runs off it or collects in little drops on it, revealing the image you created with the wax.