Plotting the campaign
Alright! So if you're following along with this series then you have a theme, you know some details about your setting, and you know what your players are being, now it's time to put it all together. Just follow these five simple steps:
Step one: Start with what you know. Look at all the little tidbits and details you've set aside and grow them, branch them out. If you introduced a guild or faction then consider where they are, how they are, and how can you invert the idea behind them to make rebels, former members and the worst of the organization. If you know two countries are at war figure out how they do battle, why and where. You want to give yourself a bunch of information and then separate out the sensational bits. Figure out what you want to tell players, and what you want to show them. If you can sum up an organisation with a few quick words that's great, but if you can captured the detailed essence of them with an adventure, thats better.
Step two: Connect the dots. Once you have an idea of where the juiciest bits of your setting are you need to figure out how to get the players there and how to make the most of those areas. Look at their motivations, their classes, their purpose, and the world around them and build a framework for creating a smooth path that leads across the best of your setting. Make sure that everyone has something to do at these major areas. If one player is an industrious merchant than definitely have a market and guilds to barter with, but don't leave the fighter and the wizard hanging with nothing to do.
Step three: Create speedbumps. The campaign will need some opposition in order to keep interesting, but don't mistake opposition for obstacles. I use the term speedbumps because when you're traveling down the road the speedbump makes you slowdown and change up a little bit, but it doesn't stop you from going where you want to. A group of rival adventurers isn't likely to stop players from completing their quest, but it will certainly make them slow down and deal with the problem. Conversely something like the port town having no boats to take them across the sea, or the next mcguffin being kept in another dimension are more likely to be real roadblocks, at least for a while. When the party isn't sure how to proceed it can create confusion. It's important to make it clear that they can proceed and give them at least one way to do so. These speedbumps can be small adventurers or planned encounters and each one should have some purpose or convey some message besides just "kill everything bad".
Step four: Read yourself the plot. Does it make sense? If you were reading or watching it would youenjoy it? And then go through every way your plot can be circumvented and have at least some idea of what to do about it. It's going to happen and that's alright. Nobody can keep a group of empowered excited players in line and you don't need to all the time. Sooner or later they'll probably return to the main story, and if they don't then it's because they're having too much fun doing something else.
Step five: Editing. Look through the plot you have planned, does it fit your theme? Does it include the whole party in multiple places and tie all of them togather? Can you introduce new players part way through if need be? Can the plot survive loosing any of the players? Run through these basic questions and make changes as follows.
Follow these five steps in as much or little detail as you like and you have the structure of a campaign plot laid out in front of you. As far as the actual writing, I'll leave that to you.