Now, straight away - spray the surface with the water bottle. You want blibs and blobs of water but not huge patches of it. This needs to be done straight away before the paint isn't totally dry.
Now count the one potato, two potato, three potatoes till you get to twenty, then scrape the water off the same way you applied the paint. Wherever the water fell will come away giving you a lovely splotchy effect showing up the light colour you started with