Start by collecting photos of the city that you're interested in. Building details as well as overall city layout are important. The internet is a vast resource for this and I found lots of good pics at www.corbis.com . Find a photo that is at least close to the view you are looking for (see pic right). This is where you start your design.
Then, using this picture or set of pictures as your basis, you'll want to tailor it to your needs. Some things that you'll want to look at are scale, size, perspective, and complicated details.
Scale is important because you want to simulate the proper distance to the skyline. For example, if the model is too small, it will appear as if your house/apartment is too far away or too high up.
When considering size, realize that as you approach your window, your field of view widens. So you actually need to make the city larger than the window if there is going to be any distance between the city and the window. Distance between the city and the window increases realism in two ways. When walking past the window, you want the city to shift in relation to the window frame just like it would with a real city view. This is a natural effect called parallax that your brain uses every day to judge distance. The other reason for placing the city out several feet, is to minimize your ability to use stereo vision to see the distance to an object. Human stereo vision is only effective to 12-18 feet.
Perspective is the feature that will really sell your city as a realistic view. Notice top to bottom perspective as well as depth perspective. Top to bottom is pretty easy.. mount the city so that the average person's eye height is going to be level with the floors of windows that do not slope up or down. Mine is slightly off center top-to-bottom in order to fit more skyscrapers in, but it isn't obvious to most people. You'll need to make a judgement call on how much depth perspective to use. Was the original photo taken with a really wide angle lens?
You will also want to choose a photo without complicated details or edit them out. If you're an expert model maker, then by all means build a Brooklyn Bridge. It would look fantastic! But the rest of us will probably want to tone down the complication. This project is going to be laborious enough as it is. Avoid street-level views because the streets of a city are usually very animated and a static street scene will look odd.