Before the game begins, help the players out by telling them how to create the party you need for the game. Include a more powerful NPC in the party so that you can enjoy being a player as well.
Create an adventure that involves as few choices as possible. The closer you can get to reading the scenario to your players, the better (you don’t want them mucking up your story). And don’t worry if the PCs don’t have the skills or talents that they need to enjoy the scenario. That’s what your powerful NPC is for, after all.
The first battle should be stacked against the PCs — you don’t want them to think they’re all-powerful. Skewed odds are more realistic, and they’ll give your NPC a chance to really shine.
If by chance the party looks like they’ll be able to win without your NPC’s help, you have two options. Either fudge rolls liberally to ensure that the baddies never miss (and always do maximum damage), or simply have more of them show up. Remember: Nothing builds character like losing every fight.
Clever ideas are your enemy. You can’t tell your story if the players keep screwing it up, so make sure that whenever someone has a “good” idea you’re ready to shoot it down. They’ll learn to keep quiet quickly enough. Another option is to have your NPC explain why the idea won’t work, and come up with a better one yourself.
Just like in the real world, bad things should happen to the party without warning. And just like the real world, the PCs shouldn’t have a chance to avoid them. Having someone break into their home base and steal all of their stuff is a great example — much like dropping a deuce in the urinal, this is fun for everyone.
You’ll know it’s time to wrap up the adventure when your NPC has had enough time in the limelight. Narrate what the PCs do during the climactic battle, and don’t be afraid to kill one of them off — that’ll provide some great fuel for your next adventure.
After the session, don’t forget to ask the players for feedback. They probably won’t say much (players aren’t usually very motivated), so you’ll have a chance to go over the highlights of your story again. In fact, you’ll probably need to do that next week, too — players tend to be pretty forgetful.
And next week? Best. Story. Ever. Remind them to bring blank character sheets, because only the most skilled players will be able to survive this adventure.