Below text from page 17 of Werewolves, Warriors and Winter Sacrifices: Unmasking Kivik and Indo-European Cosmology in Bronze Age Scandinavia (2022) by Anders Kaliff and Terje Oestigaard (published by Uppsala Universitet)
"One may identify four ways in which it was believed that a person could become a wolf. The most common way was, as pointed out by Brown & Anthony (2019), through the youth war bands, the kóryos or Männerbünde. The initiate was selected at the age of eight and trained for four to eight years. At the midwinter solstice, the neophytes were initiated into the war band and, in some cases, sent away as outlaws to live in the forest as wolf and dogs for four years. Second, during the harvest, the corn-spirit could embody the last sheaf, often in the form of an animal - a wolf, dog or mare. These animals were either sacrificed as part of harvest rituals to mark the beginning of winter, or, in the case of domesticated animals, they were kept in the farmhouse for use in specific rituals at midwinter solstice. If an animal was sacrificed, its hide could be used as a hood that would transform the ritual protagonist of sowing rituals the following spring. Third, numerous sources describe how humans could actively transform themselves into werewolves. One way was to pronounce a spell while proposing a toast; another was to climb through a belt. Lastly, a less common way of becoming a wolf was by birth."
