Understand the use of the prop. For example, props may have some special significance for the scene. A bottle of alcohol can be simply decoration or it may be smashed on someone's head. The use will determine which kind of bottle should be used.
Research the real thing. Before building the prop, research similar real versions of the item. For example, if a wizard's staff is needed, look at the staffs depicted in artistic renderings of famous wizards such as Merlin.
Select the materials. The materials will vary depending on the prop. The materials should always be the most lightweight, inexpensive, durable and safest possible. For example, if a jeweled box is required, do not use real jewelry. Props should be easy for the actors to handle. For example, a real Excalibur sword could weigh up to a hundred pounds but a plastic or aluminum version would weigh much less. Props should always be safe. For example, rather than using a real rock as a murder weapon, use a fake foam rock and take any risk out of the scene.
Consult with actors on the props. The actors will be the ones using the props, so get their feedback on what specifics they would like. For example, a left-handed actor who uses scissors in a scene may want left-handed scissors.
Create a blueprint for the prop and construct it.