Here is my 2 cents on how to design a indoor and then a out door level.
Indoor.
Write down everything you want in that level, monsters, loot, NPCs, traps, lights, describe the area.
Draw out on hex or graph paper the level, and mark each room, hallway, etc with identifiers (R1 or something). Then list each one, and describe the room, hallway etc.
Once done, do this.
Use basic shapes to make hallway sections, rooms, etc, then peice them together, after each one is done. Its easier to work on smaller files then larger ones.
Then peice them together in the program, or modeler.
Outdoor design.
This is what I do, this is how I have to do each area. Its involoved, but when I have 9 people take what I have done afterwards into their areas of expertise, descriptive details help.
Use graph paper, or Terrain Mage or another type of program and design the entire outdoor level, either its a island, or continent, or a whole world.
Make each section into grids, then make basic details of the area onto a new peice of paper. NOW. Describe the areas, mountain, forest, swamp, cities, etc.
Then take each grid area and begin to come up with details, lay of the land, shape, sounds, placeables, etc.
Go more detailed, with drawing, the lay of the land, where everything is. and make sure it works from grid to grid, sub grid to sub grid.
The entire land mass for the RPG im working on with Level 10, is over 300 pages of graph paper that I have drawn, redrawn, etc. Its taken months. but its so detailed, If I left the project, anyone could take it and move forward. Now the land is huge, and detailed, and larger then most would need. But you get the drift.
Its amazing what you will forget. Its best to plan now then screw up later.
Especially if you want to push the idea to a publisher, shareware publisher, etc. You have to have documentation. And it makes the whole thing easier later on, even if its a just a hobby project.