Alternate Worlds
All that is known to humanity is within the realm of our own world, and often when trying to escape the binds of nature's laws, we create our own ideas for worlds beyond - places that exist without those binds, or worlds that function entirely differently from what we've seen and experienced.
The process in which Alternate Universes are created is a fluid one, one that varies between creators. Depending on what your goal is involving an AU, or at the very least what your focus or purpose for it is, you may want to start with broad ideas such as natural laws or cultural aspects of residents, or you may want to branch out after starting with specific quirks that define what is unique in your world.