How complicated a city backdrop you make depends greatly on your resources. If the backdrop is for a small theater performance, and you don't have a large budget, paint on cloth will suffice. For a more ambitious production, you can incorporate props or costlier materials. Your city backdrop can be realistic or more abstract, depending on the nature of the production.
Find pictures of cities to help you formulate your design. Print a picture from the Internet, find one in a magazine or take your own photographs. Working from an original will help you to stay accurate. For more abstract interpretations of a city, use a picture as inspiration, but allow yourself to expand creatively.
Stretch out muslin cloth and staple it down to planks of wood at the two side edges. Ensure that the fabric is tight, then sketch the city backdrop design onto the cloth. If you are struggling with the scale, divide the design and the muslin cloth into smaller sections. Use the sections as guides to help with the sketching.
Paint the city backdrop once you are happy with the sketching. Use an acrylic paint, as this works best on muslin cloth. Paint all areas that are the same color together. Do not attempt to remix shades of paint as they may not look the same. Paint darker colors first and allow them to dry fully before painting lighter colors to prevent smudging. Use dark or light paint depending on the time of day you are attempting to replicate.
Buy or make props to enhance the backdrop. For example, use white Christmas lights to represent blinking city lights in the distance. Or, place trash cans in front of the backdrop to create a city street scene.
Hang your backdrop when it is dry, using the planks of wood to secure it to the back of the stage. Touch up areas of detail, or select more props once you see the backdrop hung up.
Depending on the setting, you can include details that refer to a specific city, for example the Eiffel Tower to suggest Paris or the Statue of Liberty for New York City. For an industrial setting, factory buildings and smokestacks set the scene.