Everyone's approach varies. Approach isn't as important as fundamentals and writing a good story. And, as always, just keep writing.
I tend to start with an idea in my head and expand on it. Just one idea that usually becomes the premise for everything else. I've outlined around an idea, written poetry and music around ideas ... you get it. I have no idea where it might occur in the book, but it will be in the book.
In any case, I have an idea and write around it.
Again, everyone's approach differs. What matters is that you start something, finish it and follow through.