X – X Marks the spot.
Note important locations, map it out – It does not have to be fancy, even a node map and rough idea of where things are will suffice.
With all of the free maps from so many talented artists and map makers in the OSR, and via many various websites, like Cartographer’s Guild, one can easily come up with a map. There are also lots of maps from the medieval and renaissance periods that are available online to give one ideas. There is more than one G+ community for maps and world building ideas. There is a surplus of riches in maps and adventure ideas, thanks to the internet.
For some city and town maps, I have taken something I found online that was close to what I had in mind, and used that to help craft something for my own needs.
Where is the important stuff? Where is the treasure in relation to the palace or the temple?
Have the rich buried or hidden their wealth when they fled, is it still there? Is it inside the city, or somewhere outside the walls?
If an abandoned city for hundreds or thousands of years, what groups, monsters, wizards, cults, etc. have made a home or base? Is that base still there? Is it inhabited by the original founders of the base or has it changed hands over the years?
All of the other articles in this series mention many different things that will have a location. Each of those things will have a quality of the original construction, degree of routine maintenance given when in use, and the use, damage, neglect, etc. that these things have undergone if the city is abandoned.