I used a black sharpie on a perfectly clean mug and then baked it for 45 minutes in the oven and let it cool for 24 hours before handling. The cheaper the mug the better because the glaze is cheaper and allows for the pen to kind of melt in when it's in the oven.
Now one word of warning - no matter what other websites claim - these letters WILL wash off! They absolutely can not be put in the dishwasher or have the letter scrubbed with a sponge. I tried everything people said to do to make those letters stay on and not one thing worked. I even went so far as to buy a sealant and put it on over the letter. That did make it stay longer, but it too, eventually washed off. So, if you want to give these out, you do have to warn the girls how to care for them.
Anyway, we attached a Christmas message to them and wrapped them in cellophane that we found for cheap at Hobby Lobby. I can't tell you what a big hit they were and super easy and inexpensive.