Cute Punny mug
My mom needed a milk mug and I needed a birthday gift for her, so this was the solution to both of our needs! jk.
DETERMINE A DESIGN. I spend a good while looking for some inspiration/good puns, but surprisingly there aren't that many about milk/cows. However, I did happen upon this comment in something and I wanted that rounded cute font, so I found those examples online. This is the one I used: http://www.fontsquirrel.com/fonts/geometry-soft-pro
PAINT. Using the ceramic paint, I just got mine at walmart for $2, go over your plan. It is ok to paint over the washable marker, so don't worry and the sketch can easily be washed off so you can change the design a little. If you make a mistake, wait for it to dry and then scratch it off, nothing is permanent until you bake it. My paint said to not put it anywhere that food will touch, so be mindful of that. The rim I think is fine since food won't be sitting on it.
DO SOME COW SPOTS. There is no science to the cow spots. Just draw blobs, but make sure there are evenly spaced and varying in size to give it the best look.
BAKE ACCORDING TO INSTRUCTION. My paint said bake at 350 for 30 min, then let cool in the oven. You don't want to take it out right away because it could crack from the sudden change in temperature.
WRAP AND GIVE. Or just use it yourself!