How does one go about establishing a parallel universe?
Establish the rules for creating parallel universes: How are they created? Does every little choice create another universe? Are they created through magic or scientific experiments? Are they created every time a star dies? Are they created every time someone dreams? Are there specific people capable to creating another universe (either intentionally or by accident)?
Establish the Point of Divergence: What was it that made one universe split into two or more? How does the POD affect the parallel universe (i.e., what are the differences and similarities)? The POD does not have to be recent. It could just be “the big bang created two universes instead of one and they are exactly the same in every way except for X”.
Establish rules for travel: How do people travel between universes? Are there any rules that people have established when it comes to traveling to another universe? What are the reasons for these rules? What are the dangers of travel? Who is able to travel? If you travel to a parallel universe, are you able to return? Are there any physical effects of travel? What happens if someone travels to a universe where they are dead?
Establish the differences: Keep track of everything that is different in the universes. Making a chart might be helpful for this. You should also consider how these differences affect other differences. Think of each difference having a domino effect.