A very good place to start...
FIRST - Today is release day! If you haven't gotten your copy of THROUGH THE VIEWFINDER, what are you waiting for? The ARC readers reporting have loved it (and I think you will, too).
SECOND - my editor suggested I take the secondary characters and write a story for them, which I am doing. Chapter 1 in any book almost always turns into exposition and backstory in the first draft. It's sort of a "get to know you" for the author, during which the characters reveal things about themselves that make them who they are. As the story progresses, many of those details get moved into later sections where they can be added more seamlessly, but for now, I'm starting at the beginning.
The fun part about writing is that once you get started, it unleashes your imagination. So even if I'm writing complete drivel, it's carrying me away with "Oh! What if this happened, or what if that happens?" "Oh, what if the plot I imagined relates to such and such?" First drafts are flights of fancy. Let your imagination carry you wherever it wants to go. You can always clean it up later.
Oftentimes, somewhere along the writing process, the story stalls out, so the more thoughts I have now that I can capture (like capturing a fly, or maybe a butterfly) the easier it will be to navigate that stall. The muddle in the middle. The "where is this story going?" If I know that now, even just a little bit, it helps motor through when things start to slog. This is especially important when you have the ideas in the first place. There are some books where those ideas are elusive, and it takes a concentrated effort to see the forest for the trees, in a manner of speaking. Give yourself the freedom to create without structure, and you'll have the bones to build that structure later. You've heard the expression, "just write." That has gotten me through more quagmires than I can count, and it's no more important that when you're first starting out.
So if you're writing, start at the beginning. Write it all down. Let your imagination carry you along, and as the story gets further along, you can shape it however it needs to go. That backstory you started out with? It will be important somewhere along the way. Write it down. You can filter it in later where it's more appropriate. That crazy idea? Write it down. You can discount it later if it doesn't make sense, but if you don't write it down, you won't have it for later in case it fits in.
So excuse me now, as I go to see how many people are buying the new book because YAY! It's release day. Your support is what keeps me motivated. Don't forget to tell all your friends about this great new book you heard about. In between checking sales, I'll be writing the new book, because I have some great ideas for those secondary characters who are stepping into the spotlight!