Starting with the second act in a story seems much harder than starting with the beginning or even the end, simply because you have no setup for your characters or locations. I think that with a prequel story it’s easiest to start at the end and reverse engineer the story. It’s difficult since you have to think backwards, but it’s similar to thinking forwards with a regular plot. For example, Anakin ends the story as a fallen Jedi who nevertheless still values order and justice, as well as loyalty to the Emperor. Thus a potential prequel trilogy could start with a rebellious Anakin who doesn’t care about the Republic. Then his character could change as Obi-wan teaches him the ways of the Force, and he becomes the model Jedi. He’s obedient to the Jedi Order but more than that to the Chancellor. But as he becomes the perfect knight the Jedi Order realizes that the Chancellor is corrupt and the Republic is fallen.