I'm far from an expert, but I can share my experience with my web series.

We broke down the plot of the entire series into different strands. Three, in our case. We structured each strand into a rough three act structure.

We figured out how many episodes we needed and figured out which threads needed to have things happen. We spent a lot of effort to make the stakes higher and finally got to nine episodes.

After a lot of rewrites and a lot of polish, we had finished scripts. And then a finished series. Which you can watch at www.bay51.com. :)

That was kind of rambling, but I'd be happy to answer any questions.