Start simple. Keep a notebook of "Worlds" that you will be adding.
Each "World" should have its own page as a minimum.
Write down what each world brings to the party, what problems that will have to be overcome.
Plot out in the timeline when each world is introduced to another.
Each world should have a general list of reactions to the others.
As you add worlds you will need to go back, and add reaction/responses to the other world's note page.
Then you will need to do the same for the individual characters POV.
If you haven't got a Headache by this point, you may have missed something.
Each "World" should have its own page as a minimum.
Write down what each world brings to the party, what problems that will have to be overcome.
Plot out in the timeline when each world is introduced to another.
Each world should have a general list of reactions to the others.
As you add worlds you will need to go back, and add reaction/responses to the other world's note page.
Then you will need to do the same for the individual characters POV.
If you haven't got a Headache by this point, you may have missed something.