Are there any Navajo tales that explain how one becomes a skin-walker? Is it an innate or an acquired ability? Can anyone become a skin-walker, or is there a hereditary element or other constraints?
According to this blog post by Adam James Jones the "Skinwalker curse is desired and acquired":
They are stories of shape-shifting creatures acrosss Navajo Nation, the 24k-plus reservation land encompassing most of northeastern Arizona and the adjacent corner sections of New Mexico and Utah. A taboo subject amongst natives, Skinwalkers are seldom discussed with members outside the tribe, and rarely even inside it. The Navajo Skinwalker legend is not unlike that of the European werewolf: A once-ordinary human discovers the ability to shift into animal form at night where his doings then become almost exclusively evil. Unlike the werewolf, however, the Skinwalker curse is desired and acquired, that is, Skinwalkers do not have the bad luck to be “bitten” and forced into the curse. Rather, they want it and are willing to perform extraordinary rites of evil in order to achieve it.
There are multiple legends behind the origin of the Navajo Skinwalker. One claims the Navajos mastered shapeshifting in order to escape persecution and relocation — the Kit Carson-led cornering of the tribe deep in Canyon de Chelly and later their forced and disastrous relocation to Bosque de Redondo. Another version relates to the Navajo belief in the Anasazi curse — that the Anasazi were responsible for the prevailing witchcraft in the Navajo tribes — and that Navajo Skinwalkers used the off-limit Anasazi ruins and grave sites to gain certain powers.
The most prominent history of the Skinwalker tells of a particular form of Navajo witch, or an ’ánt’įįhnii, called ayee naaldlooshii, translated to mean “with it, he goes on all fours.” The yee naaldlooshii is usually a medicine man or high-ranking priest who has obtained supernatural powers through breaking a cultural taboo, including murder, seduction, or the corrupting of a family member.
Upon accepting this deep and consuming level of witchcraft, Skinwalkers are banished forever from a tribe (but considering the foreknowledge of this as well the despicable acts required for the transformation, the aspiring Skinwalker surely possessed an early, pre-seated hate for the tribe). Prowling alone in the desert, a Skinwalker (and also unlike the werewolf) has the ability to shape-shift into any animal they wish, although most commonly the animal is a coyote, wolf, cougar, fox, owl, or crow — a reason why pelts of these animals are widely restricted among the Navajo.
The site Navajo Legends isn't as detailed, but also mentions that becoming a skinwalker takes the murder of a family member:
To become a skinwalker requires the most evil of deeds, the killing of a close family member. They literally become humans who have acquired immense supernatural power, including the ability to transform into animals and other people.