When the boys join the pack, do the tattoos just appear, or do they have to get it done?

Stephenie Meyer's Twilight series features the Quileute Indians, a real Native American Tribe, in a fictionalized version of their tribe. The group is explored throughout the series—most thoroughly in the second novel of the series, New Moon.
Some young Quileute men and women are destined to be shapeshifters who become werewolves at will. The shifting occurs when there is a threat to the tribe, which is caused by the presence of vampires (the Cullens) in Forks.
In the film adaptation of the novel, each member of the tribe is marked by a tattoo meant to look tribal, as though it represents the Quileute culture. Each werewolf has the tattoo, although it's not explained if the tattoo just appears or is inked intentionally. The book series does not feature the markings at all. The tattoos were featured prominently in merchandising surrounding the movie and were possibly included as an easy way for theatergoers to quickly identify the werewolves.