I started this project with quick doodle. The idea was to depict the derelict demolished city inhabited with living zombies. I decided to draw a big square surrounded with houses and a part of a blasted bridge. Such a broad space allows nice location of different characters, objects and other stuff.
Next stage of work was the search for references (I'm extremely fond of cgtextures.com resource, which features plenty of good photos of excellent quality on nearly every topic). So I picked up some photos which best fitted the idea of the work.
I drew the doodle in gray-scale mode, which further facilitated with application of photo-textures.
For multilay I used 40-50% transparency. At that stage I didn't use contrast or levels, since the main thing was to grasp the composition and balance of large-scale objects.
Further I concentrated on textures and details of the work: billboards, garbage, blasted bridge... All such things appear along the way. To maintain creativity keep checking references and noticing fancy nuances.