Okay... as a thought experiment, and because I'm bored at work (and unable to
crack the damn web filter), I thought I'd type out how I create a plot. After
all, I've heard it all before "But we're not good enough to ST" you do cry.
Bollocks, says I. "But why should we ST?" Simple. It's one hell of a rush.
Imagine sitting up late at night, reading a pulp detective novel. Raymond Chandler, say. You've read the book before, you know what's going to happen. But that was a while ago, so all you have are vague ideas about what the story ahead is going to be like and some idea of the ending. So you read on, to see what happens in between, and how the hero gets to the ending. Imagine that sense of wonder, but with other humans charting a course, defining the muzzy events you have come up with, giving form to your ideas. Sound fun? Good. Get STing.
Stage 1: This is probably the hardest step, for me. You have to have an idea. If you already have the genus of an idea, you're set. move on to Stage 2. Otherwise... Oh, gods, how do I get ideas? TV is always a good place to start, as are movies. I find other gaming books give me plenty of ideas that I can use, as do horror and sci-fi novels. Get some things in your mind that you might want to subject players to, and there you are.
Yes, I've deliberately left this part short. Everyone has their own things that inspire them, and I'm not going to tell you what they should be.
Stage 2: Write or think out the basic framework. What the back story is, if any, what you think the ending will vaguely be like, any things you want to use along the way. Don't be too specific with anything. No good storyline survives contact with the players, so don't think up what they must do beforehand.
Say the players, a group of normal humans, are investigating something. You know that in the newspaper on the desk is the location of the killer's next victim. They don't, and nor should they have to. If they don't think to check, then don't try and force them to interrupt the killer. Just ride with it. Maybe after thinking the PCs are too close, he decides to pick one of them off? You should never have written such a specific clue... and if you did, you'd better have some other idea just in case.
Step 3: Polish. Specific descriptions and moods of parts of the story, details of various NPC bad guys, maybe stats on whatever they're going to kill. Think here, even if you're going to totally wing it, a description of an underground cavern could be any cavern, so why not have one prepared? Also detail anything the players will encounter. For Garou, their sept is a pretty good place to start, and the city that they will be residing in needs a few nouns and adjectives of choice to make it seem consistent.
Step 4: Ad lib to the hilt. If something seems to be fun, and it wouldn't interfere with the back story ("Hang on! There are no vampires in Stafford! Howcome 10 of them just jumped us?") then put it in. Never tell your players
the reason you did what you did until later. This gives you time to think up a plausible excuse. Do this three or four times during a single game, and you have four more things to run as the players look into each. Nice and simple.