The following is my 9 Step System of adventure writing...
Step 1: Come up with an idea you like. Example: Orcs with bright purple mohawks.
Step 2: Refine it. Basically, add another layer or two of detail. Example: These are punk rock Orcs with mirror shades from a parallel universe; they wear black leather motorcycle jackets, carry knives and guns, etc. They want to wreak havoc, tear down the establishment, etc.
Step 3: Come up with a couple more ideas and refine those. Example: Elves with 80's hair cuts, bright blue, pink, and silver makeup wearing matching colored spandex who also want anarchy. While the fashion is New Wave based, there's also high technology like cyberspace, electro-knives, laser pistols, etc.
Step 4: Create a connection - how can these three ideas be used in conjunction with each other? Example: A future-tech mirror allows access between this parallel universe of nihilism and oppression and the D&Desque realm the players are familiar with.
Step 5: Find the major conflict. Example: The Orcs are anarchists while the Elves want order - both are trying to create change, but the Orcs don't care who gets hurt or how violent their revolution becomes. Elves temper their desires with diplomacy, empathy, compromise, etc... a slower means of change. The PCs should help the Elves capture the Orcs and get both sides back to their own universe after spilling out in this one. This dynamic reflects the greater conflict - the people versus an oppressive corporation.
Step 6: Revise your ideas to fit the connection you've come up with according to the major conflict you've decided upon. Example: The PCs find a magic mirror in the dungeon they're exploring which links to the future-tech mirror in that other universe. The weird, New Wave Orcs and Elves want to explore the fantasy world that opened because PCs tampered with the magic mirror. Orcs start destroying everything while the Elves attempt to understand this new world and want to help the PCs, work within the system, etc.
Step 7: Think of an exciting set-piece. Each session should have a really cool encounter - a place, monster, NPC, item, etc. which sets off the adventure. If your session was a film, what would the poster show, what would be in the trailer? You need something awesome to showcase. Example: Cyberspace is full of information clusters, sentient numbers, letters, and giant Star Worms roaming through the blackness like multi-hued circuit boards, at the center of which floats a pulsating jewel powering the entire network and guarded by thirteen cybernetic demons.
Step 8: Consider options - have a place for the adventure to go. The GM should have a plan... what happens next? Example: The New Wave Elves see all the magical weaponry in this realm and decide to borrow some of it in order to challenge their oppressor: an all-powerful mega-corporation. Towards the adventure's end, the PCs might go to that other world and fight against the mega-corporation's zombie soldiers.
Step 9: Run the adventure, prepare to improvise. Example: After introducing both sides, the PCs decide to let the Orcs destroy the realm because of a bitter dispute with the King. In this hypothetical scenario, the PCs also decide to capture the Elves for information and to keep them from hindering the Orcs' destruction. How does a GM handle something like this? Think about the different options available, but try not to force the adventure to go in a direction that doesn't work.
In summation, adventure writing takes awhile, so it's best to tackle things a little bit at a time. Don't try to do everything at once, and don't let too many days or weeks go by without going over your notes with an eye for refining, adding layers of detail, fixing inconsistencies, making new connections, and smoothing edges. I hope the 9 Step System makes your adventure writing easier.