The project manager usually works with other departments or teams to put the document together. They will probably use Brainstorming and Interviewing techniques to help with this process and may even build a prototype.
Regularly review estimates, deadlines and milestones to check they are still on schedule. If necessary, update the plan with new estimates and tasks and ensure that remaining tasks are necessary and estimates are still valid. Circumstances can change during a project but question every change and always refer back to the original business requirement as a corss-reference.