Pick an environment..

The first step of any map design is to choose the environment its going to be set in.

Is it in a sewer?
A tower block?
A street scene?
Alien world?

You must have a clear idea in your head as to what the area will look and feel like.

See it in your mind.

We're not bothered about specific layouts here, just the look and feel. We need this because when you start the mapping process you need to be able to use basic dull textures or development textures and overlay them in your mind with how its going to look in the finished map.

Once you can look at a basic collection of square blocks and see in your mind what each one represents in the final map, you can start building creatively and not bother too much with the detail or textures at this point.