Decomposition
First things first, looking at the shadows, the edges are very straight. what this tells me is the direction in which the light source is coming from (camera right) and that this light source is harsh… meaning no umbrella or softbox was used to soften the light.

Another thing that the shadows can tell (I’m still trying to figure this out myself, so I don’t know if there are exceptions to this)
If you look at the shadows, you don’t see any detail in them. I believe this means that ambient light doesn’t affect the image at all. That just means that if you were to turn off the flashes, you’d just get a black frame. Again, I don’t know if there are exceptions to this, there probably are, so just be aware of that.

So we have one light source figured out. Looking at the image, I would guess two more. One behind the model shooting at the wall behind her. Looking at the shadows on the wall under her arm, you can see the edges are pretty harsh. So again this is a harsh light source.

The third light source I would guess is the one that is lighting a bit of the window beside the model. Again the shadows tell me it’s a harsh light source.

So how did i do?
I actually only used two light sources. the back wall and the front main light. The third light source I guessed was actually caused by the first main light.

Why did I set up the lights the way I did?
The easiest to explain is the light behind the model. That light is used to separate the model from the background. If it wasn’t there, the image would be very different, her body would disappear in to the shadows, so it would look as if she was coming out of the shadows. It could make for an interesting picture now that I think about it.

The main light, I can’t really tell you why I put it where I put it. It’s just where I usually start out with the main light. I find that it gives a good 3Dness to the model. Shooting a flash straight on (if done incorrectly) usually kills the 3Dness.

Leave any comments or questions and I’ll get back to you!

Side note: I do wish I had lit the right side of her face more. it just disappears in the shadows too much.