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.
The business requirements state what is required but do not specify how the deliverable will actually work. So in many projects with a tangible and technically sophisticated deliverable, it is very common to produce additional documentation about the look and feel of the end product.