Exactly what do you want to mold?
I think the process is basically extremely simple but the processes to achieve the mold are determined by the complexity of the part and the materials to be molded (Plastic, plaster, etc.).
Molds to make boats or bathtubs from composites are relatively easy - in this case, the model is the tricky part - the mold is made from a model (not always but let's keep this simple). Molds to injection mold plastics or urethanes can get tricky depending on the part and the properties of the material.
Is there a specific mold you wish to investigate?