Apply masking tape around the top of the mug
Using the lower edge of the tape as a guideline, draw lines all the way around, or draw dots, or anything you like
Let the mug dry for four hours
Remove the masking tape and harden the mug in an ordinary oven at 160°C for 90 minutes
Painting your own mugs is much more fun and personal than buying new, identical mugs. Keep in mind that the mug should be grease-free when you paint it, so wash it well and dry it completely. Remember not to touch the surface to be painted and leave the edge of the mug unpainted. The mug will be dishwasher safe at 55-60 degrees after hardening.