Create a Tatoo

No need to be 18 to get those nice body-prints...

Open up a picture of someone with some naked skin (note: with naked, i mean visible, like an arm or a leg);

Choose your desired tatoo (I've chosen a cheasy tribal-thingy);

Resize it and make it fit the body-part;

Lower the opacity just a little (note: this can depend on the lightning);

Add these effects to it;

And you're done... (note: you might wanna try to play around with the settings, because you need different settings for different pictures )