Storyboards are basic structuring tools that help a writer plot a storyline. Many books have more than one storyline. Consider The Time Traveler's Wife, which has three (the current story, the time travel story, the backstory). How did this author so successful weave these three storylines together, making a cohesive whole?
It's not easy. It's also impossible for most writers to manage just by writing through the story and never stepping back to examine the structure.
I recommend first creating separate storyboards for each storyline. You need to be clear that each storyline is strong enough on its own. Many times I've begun a book and not noticed when one of my storylines dropped out midway through. Readers will always notice and disengage. So do the structure work first thing, if you can.
Natalya also has three storyboards for her novel-in-progress. The first is the storyline of her main character in the present time. The second storyboard shows the same character during World War II. She says this backstory is "an integral part of the narrative and requires its own story line vs. being presented as flashbacks." She also has a third storyboard that shows the "secondary main character or, more precisely, the main character's main helper in the present time."
Her three storyboards seem to work individually, which is great news. But she is struggling with how to integrate them.
There are only a certain number of combinations, so look first at published examples. Then, try modeling one. Take your own material and test it out using one of the structures. If you love Time Traveler's Wife, do exactly the same with your first few chapters as Audrey Niffeneger does with hers. Test it out. When does she bring in the backstory storyline, when does she offer a chapter from the female character's point of view, when from the male's? Mimic the structure and see how you like it.
If that structure doesn't appeal to you, or doesn't really fit your material, try modeling from another book you love. Usually, within a few tries, I land on a structure that suits my book. Then I go back to my own material and make that structure my own. I tweak and change until it's uniquely mine. It's a tried-and-true technique in many art forms. Remember, you're only modeling the structure, not the writing itself.
Some writers also like to choose a dominant storyline and place those chapters at pivotal moments in the book (if you're familiar with the W storyboard from my classes, this would be points 1-5). That cues the reader that this is the most important story. Usually, that story is the present time one, narrated by the main character.
Your weekly writing exercise is to try this!
If you'd like to really practice this and get weekly coaching from me, check out my upcoming online class, Storyboard Your Book! starting on January 25 for eight weeks. Hopefully, this technique can help other writers struggling with the same question.