I spent the last year producing a ten-part web series, a post-apocalyptic adventure show that's part action, part survival horror, and part spiritual journey. It was quite an experience. I'd be happy to answer any specific questions you have about the process.
As far as the actual writing goes, I think the important part is to get a good idea ahead of time for what the feel should be. Frenetic action? Slow tension build? Dreamy vignettes? Then decide on a number of episodes and length-per-episode that fits your vision. We decided to go with ten episodes of 5-15 minutes each, for our survival-adventure story.
Within each episode, my co-writer and I plotted out an Initial Hook, an Event that progressed the overall story line, and a Cliff-Hanger. We did this for a few reasons:
1. Even with such short episodes, we knew that a viewer might come across the series at any point throughout its run, and we wanted all viewers to get hooked by whichever episode they came across.
2. Online videos of all kinds are notorious for the terrible ratio of initial plays to complete views. People will give a video about ten or twenty seconds to capture their attention, and if it hasn't by then, they'll bounce. Even with our initial hook strategy, we still struggled with this, both on an individual episode basis and a series-wide basis. (Meaning that only a fraction of viewers watch the whole thing, and only a fraction of those that watched ep one made it to ep ten.)
Series is called After the Beast: The Benjamin Walker Chronicle. Watch it at
http://afterthebeast.com/ if you're interested. We're not on the Streamies yet, but we've been complimented by some reviewers for taking web series' to places they rarely go with our epic scenery, cinematography, and adventure concept.
