When I begin work on a level, I tend to follow a standardized flow of documentation -> overhead map -> whiteboxing and mechanic prototyping -> basic asset replacement and level setup -> tech support. What I mean by this I'll explain step by step.
1. The team and I will meet up with a whiteboard and tow and start spit-balling ideas. Every field of work from the team is welcome to join, as a designers job is not only to create, but combine the best ideas of all ideas they can find. Sketching on the board and drawing preliminary layouts follows shortly after, and before long a level's base version is born. Similarly when working alone I follow these procedures, just without ease of access to others.
A quick and dirty design session of Boss 2 for Malediction.
2. Following a design rough draft, i'll create a design document detailing the specifics. You can find the deisgn document for Malediction Level 2 here. Like any good documentation, it goes through a tremendous number of revisions and updates to keep the design up to date for the whole team.
3. Next up after drafting a document is drawing an overhead using that document. I build it to scale in something like Photoshop, but due to the nature of game design this tends to change dramatically once its in game and viewed from player perspective (iterate iterate iterate)! This overhead will usually be attached to the design document as well, ensuring any who read can get an idea of what they just read.
Level Overhead for Malediction Level 2.
4. Once the overhead is created, I take it into engine and start building on top of it with whiteboxes.
5. After completing the basic whitebox, i'll begin working on prototypes for any in-level mechanics. Puzzles, cutscene triggers, etc.
Malediction Level 2 in its infancy.
6. Finally once the level is stable, I'll begin replacing important set pieces with the artists creations, and aiding in the general replacement of assets.
7. Once the level is beginning to shape up, I enter "tech support" as other disciplines begin doing their duty (art replacing, programming optimizing) , I swap to work on the next level while aiding in any general task or question another person has on the level. This step usually lasts for the rest of that level's life.
Malediction Level 2 starting to shape up.