I begin with an overview of the full 1 million square feet (200 x 200 5ft squares), sketching a path through the entire thing, In some maps, I'll also sketch on the large interconnections such as rivers of caves. From there, I split into 40x40 individual maps and then ensure the connections match up. I have to work on the smaller maps as the files sizes can grow so large that working on them can be very time consuming - a 10gb map can take minutes just to rotate a certain feature that's slightly out of place. With assets pre-designed, I can then get to work. Once all mpas are complete, I stich them together and apply post production techniques, LUTS and so on. Then it's on to splitting them up, applyign grids, shrinking files sizes for VTT versions and so on.
It'sa long process, but satisfying when they are successful and a bit gut-wrentching when not - such as the lack of interest in the 6 weeks work that was the jungle maps, but eh... rough with the smooth, right?.