To make a Christmas card using this technique, you need some watercolour paints, a brush (use a big one that'll hold a lot of paint, rather than a small one), some wax crayons (if you don't have any, a wax candle will also work, though it's not as easy to draw with), a glue stick, and some card (if you haven't any card, take some sheets of paper from your computer printer).
I prefer using a white wax crayon rather than a coloured one, because then the 'drawing' doesn't show up and unless you look closely it's a little bit of a mystery as to why the paint hasn't covered the areas of the card. But that's just a personal preference; the color of the wax crayon or candle doesn't play any role in repelling the paint.
