But that's not the only thing I mean when I talk about lighting in layers. The crux is to consider each individual light source as its own layer, and wield it independently—as if I were layering paint onto a canvas. The background layer is painted with a background light, but the subject might have two, three or even more layers of light falling on its surfaces from different directions. For instance, a main light, hair light and fill light are three separate "layers" of their own, even though they're all affecting the same subject. I use a light from behind the subject (a hair light or kicker) to create an edge of highlight for separation between the subject and background, so this light must be considered its own layer—after all, it's working totally independently from the other subject lights. And any other light sources working in the scene must be consider as their own separate layers.
Now I've got you thinking of lighting as a layered process, but how do you actually light in layers when it's time to get hands on? The technique is simple: just add each light to the scene individually, one at a time, and don't add another until you've perfected the one at hand. Instead of setting up background light, hair light, main light and fill all at the same time, pick just one and start with it. You can pick whichever one you'd like. I often start with the main light, and add fill and background lights accordingly, figuring that the main light is the primary light in the scene. But I've also been known to start with the background light, and then create the key and fill based on the foundation I've created behind the subject, and I've read about some photographers who suggest starting with the fill as the first layer, which makes sense in its own way. Since the fill light is meant to establish detail in the darkest areas of a scene, by starting with the fill you can consider that the baseline illumination on which to build the other brighter lighting layers.
Whatever layer you choose to start with, the key is simple: by working with just one light source at a time, placing it precisely and adjusting its power to build the ideal ratio and shape, you can break down a fairly complex lighting setup into easy-to-understand, easy-to-implement pieces. And you won't get confused about which light is having what affect; after all, lights often spill onto unwanted areas of a scene, so by lighting in layers you'll always know what each light is doing. That makes finely crafted lighting something each of us can easily accomplish.