its all personal preference but i would suggest this....
1. using topview make all your floors to give you a birdseye layout of buildings.
2. add a simple wall to border in your map.
3. do your ground to fill around buildings
4. build up all your buildings
5. do any terrain
6. put in all your shrubbery
7. fix up your borders and add a skybox
I use real life Landscapes for my map... I usually start with the enterance of the area, and just add on to it, room by room. After I have the whole level done, I'll add things like pipes, posters, tables, doors, and windows. Do it what ever way suits you. As long as it compiles, you've done it right!