When writing down the first draft of your custom adventure, I suggest ignoring all gameplay mechanics. Don’t draw maps, don’t think on how many perception rolls should be allowed, don’t even build monsters. Just concentrate on getting the story down on paper. I know this isn’t a novel concept. People have been doing it for years.
I haven’t. Sometimes I’m dumb that way.
The problem I’ve always run into, the roadblock I keep putting in front of myself, is the idea that I must finish each part of the story as I go along. If I was writing a Call of Cthulhu scenario, I didn’t just write about the creepy folks running the feed store in some backwater town. I also had to include every spot hidden, lock picking, or psychology roll I thought might be necessary. All this ever did was slow me down, kill any creativity I had, and stopped me from ever finishing the story I wanted to tell.
Recently, Happy Jack’s decided to do a system agnostic, one page (front and back) adventure contest. System agnostic means you’re not writing it for any specific system. There are no mentions of checking for traps, or detecting magic. You simply say there is a door, or the goblet on the table is magical, and the person running the game decides how to best proceed based on the system they’ve chosen to run. I thought I’d give it a shot, sat down at the keyboard, and was amazed how I went from vague idea to complete adventure in about an hour. Granted, I spent more time revising, but the hardest part of any writing project for myself, and a lot of others, is to get the first draft out into the world. All that time spent on mechanics, contemplating dice rolls, or the hardness of a door that might be broken down, all that is just chaff until you actually have a completed story to put it in. You certainly will need to be aware of those things, but until the actual story is on the page, file it away somewhere off to the side.
For me personally, the RPG sessions I remember the most are the ones with the best stories. Sure, they had good action and blood was spilled, but it was the story that pulled me in, made me care what was happening, and gave my character the courage to march onward into the unknown. Concentrate on that part first, focus on a story that will grab your players by the neck and pull them in. Remember the opening of the Dungeons and Dragons cartoon? The part where Hank and crew are literally sucked into the world? That is your goal. Regardless of system or setting, if you take them on an adventure with a great story, they will follow you anywhere, from a dungeon crawl to an interstellar space station. It is your job to make sure you take them somewhere worth going.