Where you have learned how to paint good skintones, you might find that it gets a little boring after a while. It needs a little extra. Tattoos can be that little extra and painting them is much easier that it seems.
What I use for painting tattoos:
It's a good idea to make a little sketch of the tattoo you want to paint. Some people like to have a reference but I usully just come of with a design of my own (spirals look good).
Mix a drop of Storm Blue with a drop of Sky Blue Ink and a drop of water. Try to make a few lines on a piece of paper to see if the mix is too thick or too thin. You should get a nice straigt thin line. If the mix is a little transparent that's even better!
Paint your design on the mini. Make sure there is not too much paint on your brush. It's much easier if you can get it right the first time, so you don't have to do any touch ups! Let the paint dry completely.
Now the tattoo looks more like warpaint than a tattoo. To make it look like it's "part of the skin" mix a little of your skin highlight colour (I use Elf Flesh) with some water to make a thin glaze. Use this mix for glazing the tattoos. Usually I use several layers of glazing and it's important that the first layer is dry before you apply the next. Voila, there you go :)