Look over the script to note what prop buildings you need to build. Take down notes of how much interaction the prop building will have with the actors. For example, does that proper building need a real door for actors to use or just a painted door for appearances sake? Does that warehouse front need a working crane or just a rope on a plank. Will anyone walk on the second story balcony?
Measure the stage and how much room the actors need for performance. Take the remaining space and divide it into sections, left, center and right. Do a rough sketch of your buildings on paper with dimensions that will fit in those sections. It would be helpful to have a professional carpenter's help. Once you have the designs set out, buy the lumber and enough sheet paper to construct the buildings, plus some extra to account for mistakes.
Put on all your safety equipment, you need both safety goggles and work gloves as you will be dealing with power tools and lumber. All your assistants should also have safety equipment on as well. Cut all the boards to your specifications with a power saw and a pair of saw horses. As you cut the sections, lay them out on the shop floor in their proper placing. Attach reinforcing planks across the back to provide additional support. Use diagonal cross-members to help prevent warping from weight.
Construct a right triangle out of 2 by 4s that is two-thirds as tall as the prop building. Attach the right triangle support perpendicularly to the back of the false front. Push the prop building up so that it rests on the back triangle like a tripod. Pile sandbags on the back support to keep it in place.
Cover the front of the false front with paper sheets for the walls. If the false front needs a door then install the hinges and door now. Once the prop is fully built and covered with paper, then paint the exterior. Make sure to lay down a drop cloth around the false front to prevent the paint from staining the floor.