How do I prepare a plastic mold to easily release beeswax?
Sounds like you're looking for a mold release agent.
Some examples of release agents are:

Pam Cooking Spray
Olive Oil applied with a spray pump
Industrial Mold Release Agents such as McLube

All of the above mold release agents are applied in the same manner. Starting with a clean mold, spray on a thin layer of the mold release agent into the mold, covering all the surfaces of the mold. Let it sit for a few minutes before adding your compound to the mold.
Careful not to spray too much mold release as the excess can mix with your casting compound and affect the surface finish of the cast part. A light application is all that is needed.