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. After the 40 minutes is up, turn the oven off and leave the cups to cool with the oven. Once they’re cool they’re ready to use!
DIY Sharpie Ceramic Mugs and Cups - Who Needs A Cape?