asks: How do you write an alternate universe correctly?

Interesting question! The idea of an alternate universe is that there is another reality existing alongside our current one. There could be one, or there could be many. The possibilities are endlessly mind-boggling.

Here's the thing: There is no correct or incorrect way to write it! This is because there is currently no proof that alternate universes exist, so we can hardly state what the rules might be. We can only speculate what they might be like.

The most successful ones created in fiction do it like this:

1. Consider the current reality and the elements that are relevant to your plot. Let's use the classic Cinderella story.

2. Now think of seemingly minor changes that could have happened in the alternate reality that might drastically change the story. For example, Cinderella's father never died. Or he never remarried. Or Cinderella never lost her glass slipper. Or the prince's brother decided to visit and fell in love with her first.

3. The alternate universe has to be recognizable. Having aliens land in the alternate Cinderella universe and start kidnapping people might be too drastic a change.

You have enormous free range when writing an alternate universe, because anything is possible! There is no right or wrong. Just don't get too outlandish or readers might think it's weird. :)