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!