First off, remember the word count. Like we posted earlier, this is only going to be 3-5 pages, depending on how you count it. It's going to be small.
Keep it Simple.
Use graph paper, a pencil and a ruler. Draw light, then darken your lines. If you write, use block printing, and again, keep it simple. Just use area numbers and a few names to help orient the reader.
If you are not comfortable with your art ability, do not try to draw like a pro. Just draw simple and rely on your writing. Remember, they are going to commission a professional artist to draw the maps and artwork. Your stuff is only reference for the editors and the artists. So don't sweat it.
Additionally, if you have access to and skill with artistic software and a scanner, you can scan in maps and add text that way. Of course, software like that is harder to use than a pencil and paper. If you're not familiar with them, stick to the basics.
Rememder you are not submitting artwork. You are submitting for a writing job. Any artwork you provide is icing on the cake.
Hope that helps.