To unlock the "Bad Santa" trophy in Life is Strange 2 - Episode 2: Rules, you need to find the third optional collectible in the yard at the back of the Eriksens' house.
Sean has to retrieve Daniel from the Eriksens' house next door before Claire and Stephen discover he's gone. As you approach the house, make your way around the side of the house and into the back yard. In the middle of the yard is a snowman.
Look at the snowman. Once Sean has made a comment on its state, you'll get the option to pick up the Beer Cap for the third collectible.