What are the rules for creating a custom class?

@ChrisPerkinsDnD What are the rules / guides for creating a custome class? Is there a template to create one like @matthewmercer created?

— Fox Cringle (@foxman150) December 21, 2015

Creating a new class isn't paint-by-numbers. It's an art and a balancing act. You need to playtest rigorously. https://t.co/F9uMshSwYL

— Christopher Perkins (@ChrisPerkinsDnD) December 21, 2015