From publisher blurb:
Sometimes a campaign will have special needs for archetypes or character options not found in the existing official material. If you’re in this situation, you might want to modify one or more of the classes in the game in order to provide options for players looking for a unique twist on their characters’ abilities. However, modifying a class is not something that should be undertaken lightly, and the job requires some serious effort, playtesting, and revision to get it right. The two best ways to modify a class are to swap out some class features for different ones, and to add new to an existing class. These methods allow you to use existing mechanics as a model, while drawing upon features of other classes for inspiration.