I find it hard to plan everything out beforehand so I start with a very simple idea and then constantly refine it until I'm happy with it. The easiest way that I find to do this is to start with the outer wall and then lay out broad sections of color and use a big eraser to make the streets. Then I go back with a tiny eraser and and erase color to make tiny avenues. After a bit I get some new ideas so I start over and take my base color sections and apply my new thoughts thus refining the streets - the colored areas will become the houses or city blocks of houses. It's a very organic evolution and usually takes me a day or three to get my base set up. Next I double-check to make sure that my scale is correct...I usually start with some idea but halfway through I notice that I got sidetracked somewhere so I may have a castle where the scale is 1 pixel = 1 foot but my streets are 1 pixel = 3 feet. So I have to go back and fix everything but that's mostly me just getting stupid but I do it every single time so maybe others do it as well. Then I start adding special buildings like castles and churches and arenas and thus have to tweak everything again. Then I start stratifying the populace and this requires more tweaks and walls and parks to separate the commoners from the merchants from the nobility. Once I've got that done I go back and start with the basics of needs and put in places for food management (farms, mills, bakeries, wharfs, etc). They need not actually be their own special shape but just colored sections like the housing areas. Then comes commerce so I put some areas for markets in and this leads to refined goods like chairs and jewelry so those jobs need a place to be. Then I put in the military - they function both as police and as guards on the wall. Once I've got about as many areas detailed as I think is needed then I start the pretty work but this part is optional if you're going for a straight-forward functional map. So the process is pretty simple - chunks of color, make streets, check the scale, think think think refine refine refine, pretty work. That takes me about a month of evenings to do but one of those weeks is just pretty work.