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.
Tea Time Dollar Store Mug - OPC The Better Half. This would be a great gift for a tea lover.there are tons of tutorials for using Sharpie oil paint markers on dollar store ceramics like these, then making the surface permanent by baking.