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.
A Gantt Chart is an excellent project management tool for the scheduling, budgeting and reporting of a project. The easiest, but perhaps not the most useful, tool for creating Gantt Charts is MSProject. Each task is listed on the left-hand side of the chart with a timeline shown on the right. Milestones can be easily marked and the schedule and costs can be easily updated where necessary.