How do you determine how to structure your story?

I've been struggling to find the right format for my book. It's a collection of interconnected vignettes, each focusing on a single character, and they're all set in different parts of the galaxy. The characters also live in different eras, so the timeline isn't linear. <br><br>The story begins with a character who is essentially being told the history of the galaxy by his mentor. These flashbacks are arranged in chronological order as per in-story events, but they aren't linear. I've been trying to figure out the best way to format these flashbacks so that it doesn't confuse the reader. <br><br>I've tried chapter headers, POV swaps, time stamps, etc. to differentiate each section, and nothing has so far worked to my satisfaction. How do other writers approach formatting and structuring their stories?