There are four approaches that Game Masters can use to organize their campaigns. Each one has their attractive elements and they also have their own downsides. For anyone wanting to run their own Dungeons & Dragons the basic approach that you use will set the tone for the entire game. Plan accordingly because not all of them are as easily adaptable should your game dynamics shift once play has commenced.
The first and by far easiest approach is to use pre-made game modules. There are a great number of these available in gaming stores and websites, plus innumerable PDF files made by other Game Masters that they are willing to share. These modules outline pretty much everything the Game Master needs to know in order to run the story with a variety of players. All of the notable Non-Player Characters are there, plus the important loot and clues and all the rest. The downside is that you have to hem your players in pretty tightly in order to keep them in the story and not wandering out about the countryside. Another downside is trying to tie in somebody else’s module to fit your existing game world (if all you do is run modules then this isn’t an issue) because the modifications you might need to do could end up being more work than just doing the plotline from scratch.
There are three ways to design your campaign from scratch: Reverse Engineering, Forward Engineering, and Hybrid Engineering. Reverse Engineering is the favored approach for Game Masters that are more inclined to maintain fairly tight control over the flow of the game. In this method the Game Master begins planning by designing the major hurdles, puzzles, and villains first from the top and working down. In terms we can all understand, we’ll use the Star Wars model as an illustration. In the Reverse Engineering method the first major NPC we would design is Darth Sidious. We would then figure out Darth Vader and Grand Moff Tarkin. Further along we would design the Stormtroopers and then eventually the denizens of the Cantina where the first fight starts the campaign. In this method the Game Master is essentially starting at the top of the pyramid and working down to the base. It takes a great deal of work to do this and requires the Game Master to have a significant amount of time to get things set up before play can begin. Another downside to this method is that with the main villains in place before the story even starts it limits the ability of the players to contribute their paranoia and develop their own enemies (sometimes a Game Master has to admit that the players can come up with some damned great plot twists, and not taking advantage of these can take some of the hilarity out of the sessions).
Forward Engineering is much faster to get the gameplay started because the Game Master only needs to figure out one step at a time. Using our Star Wars imagery the Game Master first designs the Cantina and the people in it. After this they then move on to figuring out who or what the Stormtroopers are. Eventually they will have to design Darth Vader. And somewhere far down the road they will need to design the Emperor. The tremendous advantage of this method is that the Game Master shares a great deal of the plot development with the players as they try and figure out what is going on (the Game Master is often only one or two steps ahead of the players at any given point, and even that might be optimistic if the players are particularly industrious and the Game Master is particularly sloppy). The downside though is that nobody can say for certain what the ultimate goal of the story is because the Game Master is literally making it up as they go along.
Hybrid Engineering is what I personally employ. It has elements of both Reverse and Forward Engineering. The top-tier enemies are all developed well before they are encountered. However, their minions are not as set in stone. Using our Star Wars illustration I set about developing Darth Sidious first. I then set about figuring out who Darth Vader and Darth Tyranus were. I did not however develop Darth Maul or General Grievous, because I left those “job positions” open and the players themselves came up with them and they became major NPCs. I did develop the Stormtroopers and a few of the bounty hunters but it was the players that came up with Boba Fett (I developed IG-88 and of course the players never got around to dealing with him so that was a CENSORED waste of time). I did some superficial work developing the Cantina but it was the players that came up with the band, and while I had developed Greedo it wasn’t in the plot for him to die so we had to adapt to that little hiccup. The only reason there were jawas and sand people was because the players had gone off script and wanted to explore. Luckily I had a week to prepare for that. Hybrid Engineering allows the Game Master to plan extensively for some of the more important plot details while allowing enough flexibility for the players to move the story in whatever direction they wish while still remaining firmly in the scope of the overall plot.
Aspiring Game Masters need to be aware that unless they are using nothing but pre-made modules there is a tremendous amount of work involved in running any game (one to three sessions worth of story) or campaign (basically an unending story until the players all bail out). Be ready to spend hours upon hours figuring out key speeches, important personal details for NPCs, notable treasure and loot, languages, geographical features, political and religious upheaval, even visitors from outer space and extraplanar entities. The better the story the more that is going on and the more engaged your players will be. And nothing moves a good plot like careful plotting.