In Fable III you put tattoos on in the same way as you would put on clothing or hairstyles. Go to the dressing room of the Sanctuary and use the bumper buttons to change from cloths to tattoos (the [ or ] and - or = keys for the PC version). If you are asking about Fable and The Lost Chapters, you need to find a tattoo artist. There's one stationary artist in Knothole Glade. And there are many wandering tattoo artists, you can find them in forests, mostly at Witchwood.
In Fable II you must find a tattoo artist and they will apply them there. The first one you will find is in the gypsy camp in Bower Lake.