Determine physical needs for each prop. Look at the script to determine how each item is used and whether it has any particular physical function. Use this information to plan for things like weight, sturdiness and whether the item needs to work like its real-life counterparts. For example, if a vase doesn't actually need to hold water in the play, you don't need to make it from waterproof materials, but if food props are actually eaten on stage, they need to be made from real food.
Decide how each prop should look. On a basic level, each thing should look as realistic as possible, but you should also consider how the item will “read,” or how it will look from the audience's perspective. This generally means giving it large, bold details of color and line wherever possible.
Figure out what materials to use for each piece's construction and how to structure them. In most cases, props only need to resemble real item materials and textures from a distance, but you should also take into consideration factors like weight and movement. For example, if you're building a fake dead animal, real or synthetic fur will give it a good outward appearance. However, if you don't stuff it with something that will give it heft and flexibility, it won't look realistic if it is handled or moves onstage during the show.
Build the prop. Make the item as sturdy as possible, assuming that it's going to undergo a lot of physical punishment during the run of the show. Most props do.
Test the prop. Practice using the prop in your workshop in a manner similar to the way it will be used onstage. Handle it a little more roughly than you think it will be used just to be on the safe side. Practice in front of a mirror to get an impression of how it might look to audience members.