The pelt is put on the skin of the caster, or over the shoulders if it's a larger pelt. The caster cannot be wearing any clothes or equipment when performing the spell, it will not succeed if they do. Once the caster has worn the pelt and spoken the words, the pelt will melt into their skin and spread across their body. Muscles will rearrange, bones break and reform. It's an excruciating experience and the caster must make a physical save agains 4 for going through the transformation.
The caster will entirely form into the animal shape and receive their senses and some of their instincts. Luckily, they are still able to speak and think. The speech may be somewhat distorted and garbled depending on the animal, but they can make themselves understood. The transformation back into their original shape is automatic and happens either at sunrise or sundown, whichever comes first. There is no way to shift out of the animal form unless someone else casts the same spell upon the animal and also possess something from the body of the shapeshifter, such as severed piece of flesh, skin or a large lock of hair.