This is the magic
Clean your mug (or whatever) with some white spirit, make sure it's dry and then you're ready to have fun! I suspect it is best to keep your greasy little fingers off the surface you want to decorate, so be careful how you hold it. Don't worry if you make a mistake when drawing on your mug, you can wipe it all off with water!
Once you're happy with your design, leave your mug to dry for 24 hours. Then put it in a cold oven and once the oven has reached a temperature of 150°C/300°F bake your mug for 35 minutes. Let your mug cool down in the oven. And that's it! Sooo easy, right?
I'm already pondering what to draw on next. Maybe my plain green mug..! Let me know how you get on with this, I'd love to see what you come up with. :-)