Most of us have probably seen an alternate reality depicted, especially in SF. There used to be a TV show called Sliders where, in every episode, the characters were forced into an alternate reality from Earth. These variations can be easy to create. All we need to do is make some changes that have a significant impact, even if the average person doesn’t realize the influence it would have (until reading our story). For example, on Earth, what if the Nazis had won WWII? What if we’d never invented GPS? What if global warming has raised the sea level by ten feet?
These examples are extreme, but we can do variations that impact far fewer people, such as just our main characters. One who is married in one reality is not in another, or is a parent, or lost a sibling in a way that affected their outlook. When we’ve developed backstory for a character (to shape who they are), and then we want an alternate reality for them, we can change that backstory. We’re trying to change who they are by presenting them with an alternate view of themselves or their life, one that causes them to question their life choices or even their personality in their own reality.
A template in the appendix can assist with the invention of alternate realities.