craftgrrl | Fitting an a-line, bias cut skirt?

So, I want to make an a-line bias-cut skirt, that looks sorta like this:

I've been playing around with two squares of denim, pinning it togther and trying to imagine how it would work. It feels like it should be a snap, but I'm a little worried about how to fit my, uhm, rear.
I was going to just cut two identical shapes (as above), with the waist size big enough to slip in and out of, and then just add either elastic or a drawstring to fit it.
Do you think that would work ok? I'm worried that the denim would bunch funny in the back... Do i have to put darts in to fit my bum? Would the bias give the denim enough give to fit smooth over my rear? It's pretty lightweight denim...
See this is the problem with sewing from scratch! I'm so afraid to make mistakes. Usually my projs are renovations of old things that I couldn't care less about, so I'm much more balsy with the scissors! This big expanse of smooth pretty denim makes me nerrrrrrrvous... :)
Thanks in advance for your help and advice.

And also, a food craft!