Building an entire model city takes some planning, just as real city planners must look ahead in how their metropolis is laid out. But approaching the task methodically and logically will make the job much easier. A well-constructed city will impress even the toughest viewer and can stand as a testament to your model-making abilities.
Build the base of your city with plywood. If the terrain of the city is not all flat, then use Homasote, Styrofoam or other materials to raise the portions that are on higher ground. If a river runs through the city, then carve it out of the terrain, so to speak. You may just want to paint the river onto the plywood base and cover the plywood on each side with other materials.
Draw out a detailed diagram of your city on paper before you begin construction. If at all possible, use a CAD program or other computer program that will allow you to blow the drawing up to large size. This way you can actually lay your drawing out on the base of your model and place your buildings upon the outlines.
Lay out the streets, bridges and railroad tracks before placing buildings on the model. You can purchase these items at your local hobby store or find them online. If your city happens to be the same scale as one of the major model train scales, then you can use model train equipment for much of your city. Go to www.walthers.com, www.atlasrr.com or www.modelrailroader.com.
Construct each of the buildings separately and then place it into the cityscape when it is complete. Unless your city model is very small, it will probably be too difficult to try and build around other buildings and structures. Use photos of the actual buildings for reference and include the appropriate level of detail. Use adhesive such as Walther's Goo to cement the buildings together and glue them onto the base.
Add details such as cars, people and lights, and weather the models if appropriate. You may also want to put in a backdrop showing other surrounding buildings in the city that you could not fit onto the model.