Find references and document the progress
Start with placing down the basic macro elements
Validate the work through playtests with players
Iterate and define the micro elements
Operation: Infiltrate
Open sandbox level design
Single Player FP-Shooter
This is a demonstration of how I would approach my level design document. Design documents are in my humble opinion used to inform others about your design and not about documenting your own progress. The people who look at your documentation should have a decent indication what you're designing from reading it.
