The project management plan is the guiding document you'll use to manage, control, and close your project, and it's dynamic — it'll change throughout the project life cycle. The project management plan acts as an input to the initial planning process in each knowledge area.
You may ask yourself, "How can this be an input to the initial planning process if outputs from planning processes are inputs to the Develop Project Management Plan process?" The answer is that developing the project management plan is an iterative and ongoing process.
After you have your project charter, you can begin planning in earnest. The initial project management plan could be a template from your organization, a project management plan from a similar past project that you will tailor to meet the needs of your project, a table of contents, or even input from a policy that defines how to develop a project management plan or has a list of the necessary contents.
Whatever the case, this is your starting point:
First you must define the life cycle you'll use.
You want to identify the phases and the key deliverables that will result from each phase.
Then you consider the approach you'll take on the project.
Think about whether you need a virtual team, outsourced resources, a co-located team, a beta deliverable to gather feedback, and so forth.
Finally, you identify all the subsidiary plans needed for your project and start to create a table of contents that meets the needs of your project.
For example, you may need a robust risk management plan, but perhaps you need only a few paragraphs for a schedule and scope management plan. Perhaps your organization has a policy that defines the change management process for all projects, in which case, you would refer to the policy rather than re-create the change management plan in your project management plan.
After you do the upfront work of defining the life cycle, approach, and table of contents, you're ready to start developing the subsidiary management plans that will become components of the project management plan. As each subsidiary plan is created, it is integrated into the project management plan, and the project management plan becomes more robust.
Depending on your project, developing the project management plan could take days, weeks, or months. Therefore, you need to make sure you develop a way to track the latest version, even if it's something as simple as putting the date and time in the footer. At some point, you're ready to baseline your plan with all the approved and up-to-date subsidiary plans as well as the scope, schedule, and cost baselines.
At this point, you have a document that will guide your project. Any changes to the subsidiary plans or the baselines will need to go through the formal change control process, and the project management plan will be re-baselined with any changes incorporated.