Read through the script and make a list of the props you'll need to build.
Design two-dimensional props that look three-dimensional. For any shots in which a specialty prop is seen at the background (from a distance of at least 10 feet), is not moved or handled, and filmed from a fixed angle (where the camera is not panning around it), you can substitute a high-quality, life-sized glossy color photo mounted on cardboard. This technique works especially well for items seen through windows or other clear glass. Consult with the director of photography as you plan these types of props to see what you can substitute.
Cover props with an external color and/or texture layer. After you have designed each prop to give it the proper size, shape and weight using the modeling materials you have available, give props an external paint job or apply a coating material (here, again, you can use textures printed on photo paper) to make it look authentic.
Brainstorm on ways you can create different props using the same materials. Divide the props on your list according to their physical properties (e.g, hard, soft, flexible, stretchy) and determine which similar items can be made from the same arts, crafts and building supplies. Look to work with materials that are as versatile as possible. The most common supplies include hardening modeling compounds (such as polymer clay, air-dry clay, or moldable craft plastic), papier mache, wood, upholstery foam and cloth.
Create props made from altered secondhand items, where possible. Browse thrift stores and garage sales to look for inspiration and discover objects that can be transformed into the props you need. For example, you might create a medical IV bag by combining surplus medical tubing with a thick plastic bag, or you might make a crystal ball by removing the dried flowers from an old-fashioned glass globe flower arrangement.
Use cheaper, fake materials and textures that will look like the real thing on screen. For example, use plastic or vinyl leather in place of real leather, or use items painted with high-quality metallic spray paint instead of real metal.
Add weight to lightweight items using steel or lead shot (very small metal ball bearings) or other metal pieces. With many props, the difference between the appearance of a high-quality, expensive prop and an inexpensive homemade item is weight. Fill soft goods, such as baby dolls, with a partial stuffing of lead or steel shot (or other metal objects, such as coins or nuts and bolts). For hollow, hard goods, glue the metal hardware to the inside of the prop. For example, if the script calls for a large gun, and you've built one largely from PVC pipe, glue hardware to the inside of the pipe to make it appear a realistic weight when the actor handles it.