Plans, by their very nature, are dynamic. The process of developing the project plan is dynamic if not sometimes chaotic, as can be seen from the many interleaving elements that force a balancing act. But once the plan is finalized, a review process should be developed and strictly adhered to. This is the time to get all stakeholders on board with the project and your approach to accomplishing it. Note that a plan is never final, nor is it ever completely accurate, at least at the stage before the project is implemented. This is because risk events occur, customers change the project scope, some task was overlooked, technology becomes obsolete, and the technical approach may not work as planned. Some or all of these things happen in every project, and that is why a project plan requires continuous updating.
The review process can be done with individual stakeholders, which can be time-consuming. Or it can be effectively done as a briefing or a series of briefings to the collective body of stakeholders. Usually, the latter is preferable because not only is it more efficient, but it provides a forum for discussion, which usually results in uncovering other concerns and issues that need to be addressed. In the end, all the stakeholders internal to the organization have to be convinced that the project is one that they can support and that supports the strategic goals of the organization. Once the internal stakeholders sign off on the plan, the customer then should also sign off. Usually, no one literally signs the project plan, but there is agreement, and official, written notification is provided to the organization (from the external customer) and the project manager. This is the final step before the project actually is implemented.
Project implementation is the actual start of project work. Although a lot of data gathering and even some high-level design work is a part of the plan development, the start of deliverable production can only begin when the plan is approved. Project implementation is usually officially acknowledged with a kick-off meeting, and the customer is invited to attend. After all, this is when we find out if the planning cycle was successful.