You can find instructions here, on the Design Mom website.

I didn't follow the directions to the T, but not because I didn't want to.

1) I'm not sure what the state of the local Goodwill is in your area, but ours (multiple ones) did not have any plain white mugs. At all. The end.

2) I didn't have any transfer paper (but mostly, didn't feel like going out to buy it) so I simply found a font I liked, and traced it onto some regular college-ruled notebook paper (thin & can easily be placed on the computer screen for tracing). Next, I taped the paper to the cup, and traced the outline of the letter using a Sharpie. I made sure to allow the ink to seep through the paper onto the cup. It worked like magic. Then I simply used the paint pen to put into effect the trace pattern. (If you mess up, the Sharpie can easily be cleaned off the surface of the cup using a Lysol dissinfectant wipe. Or you can wash it, as Gabrielle mentions in her post).

3) Because Juniper will give these to her teachers, I added a little personalized inscription on the bottom so that they can remember the little sprite that gave them the best coffee mug they will ever own.