I am not sure if these steps are exactly where I want them just yet but here is a rough guideline to making a one-sheet. Any feedback or disagreements are appreciated.

10 Steps (just like character and demon creation )

1. List your influences and inspirations and keep them in mind during the process.

Re-watching or re-reading your influences can be cool, especially if you can screen the movie or lend the books to the players who will be playing.

2. What is Humanity?

This is the engine that drives the game. Without it, the paintjob might look pretty but it won't go anywhere.

3. What are Demons?

If Humanity's the engine, Demons are the tires.

4. What are Sorcerers and thus what is Lore?

What the players will be is important, they need to understand who they will be portraying.

5. What are Binding, Summoning, Contacting, Punishing?

6. Descriptors, do you need new ones and if so, how are they informing the players about and marrying the PC's to the world?

These are important, they are how you display the world. During character creation, I would show them the one-sheets and then while they were still percolating with vague ideas, I'd shove the descriptors in their face and watch their eyes light up.

7. Give it a look over. Is it fun? What were you trying to do with it and what has it become? How do your rules choices reinforce the feel you are trying to achieve?

Reading other Actual Play posts (especially Art Deco Melodrama), re-reading Sorcerer, Sorcerer's Soul and Sorcerer and Sword are good ideas at this point.

8. Get feedback from others.

Post it. Show it to friends. I find this step invaluable. Very often I have a kernel of a good idea and I'm not even sure what it is. Getting that feedback is invaluable.

9. Make a one-sheet.

This is a blast, it is like making a movie poster for your game. On-line resources, clip-art and fonts can make 'em really nifty. All of your players will have this in front of you. I have put the quote from the Sorcerer rule book, "During combat, role-playing rules the dice!" on all of my one-sheets, so players remember that when they describe stuff, they get more dice and more dice = more power.

10. Play. Show it to your players, make characters. If there are still any leaks in the dam, this will be where it shows. Don't forget to post how it goes.