Back when Lamott was writing this book in the '90s, the Backstreet Boys were new and people had Polaroid cameras. Now Polaroids are called Fujifilm Instax, but the idea is the same: you snap a shot, and the camera pops out a photo that develops right there in front of your eyes.
There's a short time when the photo is developing and you can see it taking shape, but you don't know quite what will come out.
Lamott says writing is like using a Polaroid. First you just point the camera at something and take a snapshot. In the last chapter, she pointed the camera at her school lunch bag, but as the picture developed, she noticed the boy leaning against a fence.
Anne gives lots of examples of other things an author might see as the Polaroid develops, and we can see how things that weren't even noticed at first might come to define what's important in the picture.
You can't tell at first how a piece of writing will develop. She says, "You just knew that there was something about these people that compelled you, and you stayed with that something long enough for it to show you what it was about" (6.2).
Lamott launches into a long and cool story about how one of her articles developed like a Polaroid.