[It may be imagined that, once the basic elements are decided on, the GM should simply begin mapping the project on whatever scale seems convenient. On larger projects, however, it is often useful to begin with a “pattern map”; that is, a plan of the pattern by which the whole will be mapped.
That is a decent start, but its not what I thought you meant by this. I think you've placed features on your map too soon.
I would have become by imagining the major streets of the old city. These streets would cut the city into districts and mark the place gatehouses about the perimeter. Consider what might have made each street remarkable. Did it have triumphant arches and monuments along it? Was it lined in its day with trees? Was it split down the middle by a park of some sort where obelisks, shrines and memorials might be placed? Was it lined on either side by statues, icons, or lanterns? How was it paved: flagstone, bricks, great paving stones moved at great cost? If not well paved then, it might be noted now only by being a relatively impassable mass of plants pierced only ocassionally by viable passages. Conversely, if made of blocks magically fused together, it might still provide a clear road and passage deep within the ruins (and hense would be watched by many residents). What made the street memorable in its heyday?
After drawing the major streets, I would imagine the districts, placing a government district, a temple district, a market district, a harbor district, an upper class, middle class, lower class district, and perhaps districts of special interest like a foreign quarter where aliens to the culture lived (homes of dwarves or elves or minotaurs or whatever is appropriate to your town).
I would then consider the connecting streets that allowed access to the district. Districts like a market district or government district would likely have one or more large streets that ran from the major streets into the middle of the district. Lower class districts on the other hand would generally not have such streets, with the interior of the district becoming increasingly a warren or maze of winding narrow alleys.
Since your city is ruined and you don't wish to map all of it, you can probably get away without mapping the tertiary streets.
Only after this is done would I go about placing the major landmarks.