Step 1: Stencil a mustache (or whatever you wish put on your mug) onto the mug with a sharpie.
Step 2: Bake it in the oven for 30 minutes at 350 degrees.
Let the mug cool, and then you can use it. I would suggest washing it by hand before using. You may want to wash it by hand all the time, just to make sure the stencil doesn't come off.
I got the idea from A Beautiful Mess.