The work breakdown structure (WBS) should define in some detail (cost, resources, time in small enough chunks to estimate) the work of the project. The estimates should be made by those doing the work. With this information, the next step would be to begin a project schedule by entering the tasks and adding the dependencies. You may also modify the plan to avoid risks, negotiate changes with the team to meet goals, and negotiate changes with the project sponsor.
By taking the time to complete a WBS with good estimates, you can build a solid plan to manage that you have a good chance to meet.