Two different animals. One’s golf, the other’s a game of pool. I’ve never written anything but straight-up novels. Haven’t even managed to write a decent short story. Novels however, I can do. You start at the beginning, grab a handful of characters, throw them into trouble and then just keep the camera on them as they race toward the end. Give them some authentic motivation/personality and they will take the plot where it needs to go, surprising you in the process. There will be some bad guys. There will be a subplot or two. A couple of fight scenes, some downtime where they talk about their families and hobbies and become real people, and then a rousing finale that wraps the whole thing up. 90,000 words, say three or four weeks of writing, and you’re done.

Now, a series. Hootenanny. How do you write one of those? I’m not talking a trilogy, or series of novels or nothing. I’m talking an actual series, like what you get on TV, like what Charles Dickens used to do. A written tale that’s delivered in installments.

I’ve been trying to figure it out. First, you have to widen the scope of the narrative. Wide angle lens, no tunnel vision. You need to have an ensemble cast. Say five important characters that the audience cares about, and then a good dozen or so supporting dudes that flesh out the world, each with their own mini-subplot and character. You need the plot to be a slow burning one, that develops in subtle steps as threads are interwoven. You need it to be character driven, because nobody will stick around for a plot that complex alone.

So. Create a world, a setting. Come up with two or three groups that are in conflict. Find the moment where that conflict comes to a head, and start there. Then trace the repercussion of that instigating moment wherever it may go. Each group reacts to it differently, and you interlace scenes with each group so that we constantly shift from one to the other. There needs to be a balance between plot development and character development, which need not be mutually exclusive. The characters should reveal themselves through pursuit of the plot, but may also benefit from carefully chosen personal scenes that are solely about them and don’t touch on the plot itself, so that a ratio of say 80% plot scenes and 20% character development scenes can be hit.

Now, you can’t just have people reacting to that one instigating moment of original conflict, powerful as it may be. You need to reflect how random life is by introducing new plot elements that force new reactions.

So, for example:

Say we’re creating a science fiction series. You have three groups:

  1. Mercenary bounty hunter crew
  2. World government agency that hires them
  3. Criminal underworld 
Now, the three of them are in conflict. The bounty hunters deride the bureaucratic government that hires them and scorns the criminals. The gov resents their need for the bounty hunters and hates the criminals. The criminals fear the bounty hunters and feel contempt for the government. So where do we start? What’s the instigating incident?
We could say that the bounty hunters take the initiative and arrest a head criminal that is secretly in league with some top dogs in the government. All of a sudden the shit hits the fan, as the hunters find themselves being lambasted for their initiative, the government becomes divided as lower rung people with integrity start suspecting their bosses, and the criminal underworld enters crisis as several power players move to replace the leader. Switch from one group to the next as they all react.
Say, two or three episodes later the criminal boss is released. The hunters swear revenge and start going after him and the top government dogs who freed him. The gov group begins to implode as people turn against each other, while the underworld suddenly has two bosses fighting each other down.
Another three episodes, and everything is getting crazy when we introduce a completely new factor: say aliens arrive and begin to invade. Or the citizens, upset and furious, begin to stage a revolt against the government. The three groups have to find their own ways to survive this crisis, as different members of each reach out to the others, alliances are formed only to be followed by betrayals, all of it driving toward the climax by episode 16 or whatever.
Throughout this series you develop the main five characters, the 20 supporting characters, give each moments to develop their own personal problems and desires, and have a number of them be killed, become traitors, become heroes. 
Finally, you pay attention to key themes that you develop throughout the series such as: ‘organizations stifle progress and lead to corruption’, or ‘there is no good or evil, only selfish individuals trying to come out on top’ or whatever.
This all takes some planning. You need to flesh out your groups, your main characters, and have an outline of where the series goes. Then you need to plot sixteen beats or whatever that will comprise each episode, schedule the scenes so that each group gets its fair amount of air time, and balance the focus of plot vs character development.
At least, that’s how I think it would work. Think about your favorite TV shows. Sopranos. Battlestar Gallactica. The Wire. Does my model hold true? Am I missing anything huge?