Paint pen or oil based sharpie,allow them to dry for at least 24 hours. Once 24 hours has passed place them in a cold oven (this is important! you want them to heat up with the oven). Turn the oven to 400 degrees and set a timer for 40 minutes.
Tips and Tricks for Sharpie Marker Mugs. A must read if you plan to add a sharpie design to a mug! Lots of great info. Sharpie marker mugs - dollar store mugs with sharpie markers. bake 350 for 30 mins to seal