According to the PMBOK Guide, the project manager should develop the project management plan by integrating the subsidiary plans and baselines from the different knowledge areas. The project management plan defines how the project will be executed, monitored, controlled, and closed. However, before developing the project management plan, the project manager should first establish the project's business value and scope, which are the key inputs for the project charter. The business case and scope document are two essential documents that provide the justification and description of the project, its objectives, deliverables, assumptions, constraints, and success criteria. By developing these documents first, the project manager can align the project with the organizational strategy, secure the project authorization, and guide the subsequent planning processes. References: PMBOK Guide, 6th edition, page 89-90, section 4.1 Develop Project Charter; page 95-96, section 4.2 Develop Project Management Plan.