Developing the project management plan is the key to project success. The project management plan defines how the project will be managed, controlled, and executed. You could think of it as a project map with detailed instructions of how to get to the destination. It basically states the who, what, when, where, and how of the project. It should be an official document that is not open to interpretation. Comb the document for ambiguities and omit or restate them.
The project management plan encompasses all other planning components. It may draw from all other processes, but the deliverable is one single document to become the project plan. For instance, the project plan will draw from the scope management plan, the cost management plan, and the risk management plan to list a few. You can see how the project management plan is the controlling document because it contains all controlling plans.
It is important to note that the project management plan is not official until it has been approved. It should be reviewed and approved by not only the project manager, but also the project sponsor and managers that will help in the execution process. The key stakeholder who is paying for the project does not formally approve the plan. They accept the plan by signing a contract. They should have faith in the firm that put the project management plan together that the client is in the project plans best interest.