Your tattoos only protect you when they are uncovered, so you gotta run around showing a lot of skin so everyone knows how cool you are. You get the basic (+2 AC) tattoo for free if you are a barbarian using the unarmored defense class option but must use your tattoo score instead of your Constitution bonus from now on.

You can get more tattoos, but you'll need better and better tattooists for the new ones to be effective, and finding a grand master tattooist is hard, and the right to be tattooed by one is worth shedding blood over. The new tattoos must represent one of your great deeds.

Let's see a price chart. Tattoos are never going to be quite as effective as plate, but they don't weigh anything, you just gotta scrap off that mud and show em off. You also get an extra bonus to intimidation/persuasion type rolls corresponding to the level of your tattoos. This represents the fear and respect your mighty tattoos instill in strangers.