Things You'll Need
Instructions
Gather together the players in your gaming group and find out what kind of adventure they'd like to play --- your work may be for naught if you create an excellent adventure only to find that your regular group has no interest in its setting or theme. Imagination is the only limit to the scope of your adventure, so you can design a game that focuses on anything from a basic dungeon crawl for treasure to an intrigue-rich political assassination to a divine mission to kill an evil god.
Using your interests as inspiration will make the adventure more fun to write. For example, if you have an interest in Roman or Greek history, your adventure can reflect that with elements derived from the Spartans' battle against the Persian Empire at Thermopylae or the navy-oriented Punic Wars between Rome and Carthage.
Choose a setting in which to place your adventure. "Dungeons and Dragons" features many official settings --- Al-Qadim, the Forgotten Realms, Gamma World and Ravenloft, to name only a few --- and each has its own themes and special rules. Choose the setting that best fits the adventure you have in mind. The Wizards of the Coast product catalog has a full list of campaign settings and adventures, but you can also check with your local gaming or hobby shop for campaign setting recommendations.
Decide on a difficulty level for your adventure, keeping in mind the levels of your group's player characters --- don't set a group of level 3 thieves against a high-level monster such as a chromatic dragon. When thinking about the trials your characters will face, refer to the monster guide in the "Dungeons and Dragons" core rules and choose enemies whose stats won't overwhelm your group in battle. Also remember your players' non-combat skills when planning puzzles and traps --- don't concoct something that would be inescapable by a band of adventurers at your group's level. Refer to the dungeon master's guide section of the core rules for more information about designing these features of your adventure, or see the Wizards of the Coast design and development blog, which features a wealth of information about what goes into creating a game while also encouraging adventure writers to keep things simple.
Plan for the unanticipated. The glory of role-playing games such as "Dungeons and Dragons" is that such games take place largely in the imagination of their players. Because of the freedom this allows players, they may well --- indeed, may often --- decide on a course of action you didn't expect, and for which you're not prepared. However, a good dungeon master always uses her narrative skill to put her players back on track without resorting to deus ex machinae. Use multiple narrative threads in your story, so that if, for example, your group kills their patron before being given a quest, some later event can help them reach the same destination without your having to shoehorn the adventure.
Make your adventure worth your players' while. Every player should walk away from a "Dungeons and Dragons" adventure feeling like she's accomplished something --- and, most of all, like she had a good time. If your adventure sees your players storming a fearsome dungeon over several long gaming sessions, consider what might be an appropriate reward to "give" your players at the end of it all, something that will confirm the adventure's importance in their minds and continue to give it weight in their estimation after they've moved on to other adventures in other places and future times --- perhaps something they'll immediately start thinking about using in future adventures, or something that will tie their identity to their finished quest, such as a special piece of equipment. Tying the end of your current adventure into the beginning threads of the next will keep your players coming back to the game session after session.