I write the plot (big picture, few details just the big key points), sketch out a map (making sure I have the key elements detailed), flesh out the plot & map simultaneously.

The Big WIP project I'm working on is actually digitising a map I did across 3 or 4 A4 grid-paper sheets about 8 years ago. The guys I was playing with loved the plot and I wasn't up to speed with computerised drawing at that time, so they were living with my "pen line and pencil line" maps on grid paper. Super-lo-fi, but effective.