What's the best way to do tattoos?

I've never done it myself, but I have a few friends who have. I've noticed that they tend to use model paint or something else that doesn't require a sealant to keep it on the action figure. Also, since you're looking to do smaller versions of detailed work, you want to find a few small detail brushes, I've seen some that are as small as a bee stinger or smaller. If there's a ceramic shop near you, they would carry the detail brushes if you can't find them at your local hobby shop. Also, I suggest practicing on paper or something else before you work on the figures.