STEP ONE: BASIC PLOT
In any case, when creating an adventure, I typically find a quiet place—bedroom, living room, seat on an airplane or in an airport (with ear-buds in) and start scribbling notes on paper. Sometimes I'll listen to music (the Star Wars Soundtrack—for inspiration), but other times, I prefer silence. I start with a very basic plot idea—often spawned from movies or TV or books or even other adventures I've read before. But just as often, the plot is simply a continuation of one of several different plot lines already running through my campaign. My players and their characters have a lot to do with formulating this overarching plot. Actions they take, or have taken in the past, along with what I know of the personalities of those involved often guide me in this regard.
I'll use an example of an adventure I ran some years ago to illustrate this process. Here, the overarching plot idea was to introduce a new threat to the Star Wars galaxy—one that could put old-enemies on the same side. The basic plot was this, then: Agents of a powerful new enemy were planning an invasion. To prepare for it, they would launch a series of attacks designed to sew confusion and drive a wedge between the various powers of the galaxy to prevent a unified defense.
STEP TWO: SCENES TO TELL THE STORY
The next step is to brainstorm dozens of scenes that could work in that plotline. For example: Wouldn't it be cool to have the players attend a fancy government reception type deal where they can rub elbows with famous NPCs—even including some former (or current) enemies? I'll jot that down and a string of other basic ideas, then start connecting the dots, from scene to scene, idea to idea and see what works to tell the story. Alas, some scenes have to be cut, because they just don't work within the plot or add too much complexity to it.
Some of the ideas I came up with in my example were the aforementioned party (an historic meeting between the New Republic, Empire and Corporate sector to sign a peace treaty). An assassination attempt (staged at that very party). Strange markings on the attackers that only a historian/scholar type working at a library can discern. A dangerous trek to an ancient ruin. Discovery of a secret enemy base with threatening technology. A foreshadowing of a giant fleet to show the true scale of the threat. These, of course, were the ideas I incorporated into the adventure. There were many that got left on the cutting room floor.
STEP THREE: FILL IN THE DETAILS
Now that I have the plot line and some scenes to string it together, I'll begin to flesh things out. Fix some details like NPCs and Stats. Names and locations—things like that. This is where the real 'work' of the creative process comes in. Or at least, that's what I consider it now. I used to LOVE coming up with incredibly detailed stats on NPCs and Equipment and the like, but now I get by with just a jotting down of major details and wing the rest. It is MUCH less work this way, and honestly? From the player side, it doesn't make much difference at all. They could care less how high of a Bureaucracy skill the Nagai secret agent has—and for that matter, I don't give a damn either! Only detail what you think is necessary, anything else you can delve into if it becomes an issue.
In my example, I had to come up with basic stats and rules for the bad guys: The Nagai agent, his equipment and his henchmen. To throw a twist into things, I stole some ideas from various sources. The Henchmen were based off of the bounty hunter Durge (as seen in the Clone Wars cartoons)—only taken a step further. They were colonies of symbiotic, worm-like things that ran around in armored, humanoid shells. They could regenerate from many injuries and weren't hampered by wounds until the fatal blow was dealt. Likewise, the Nagai was both highly skilled and had that badass ability whereby each injury ADDED dice to his skill instead of subtracting them. So the more wounded he was, the more dangerous.
STEP FOUR: THE WALKTHROUGH
This is probably the most important step—and the one that can be used to keep a story adventure from becoming a railroad. When you've got your plot, scenes and details together, you play through the adventure in your head—trying to do so from the point of your players and what you know about them and their reactions. It is HERE you will hopefully find any holes in your ideas and fix them. It is ALSO here where you can begin to plan for the tangents and curve-balls your players are likely to throw at you. Being flexible and able to 'wing' things is good. All GMs should develop this talent. But it helps to be prepared. Try to think of ways your players may react and figure out what you might do if they act one way or another. In some cases, this may mean crafting different paths to the same objective. In others, it may mean that they skip an entire scene. As a GM, this may almost seem 'disappointing', but I feel its better to let a story progress organically than to overtly force players into a course of action all the time. Subtle nudges are good, though. I've spoken on this all quite a bit in previous posts, so I won't go into it here. If you're interested, check out this and this.
So anyway, for what it's worth, that's my creative process for coming up with Star Wars adventures. Now, I'd better get cracking on actually DOING that instead of talking about it.