The basic thinking around a Waterfall project is to have a checklist of things needed to be handled before each gate (step in the process).
The steering committee controles that the checklist is performed and approves deviations to the checklist if needed.
Development is performed simultaneously to all functions and parts of the project.
The activities in the checklist can be very detailed meaning that this way of steering is quite detailed
 
The Waterfall model is an ideal model so when you run a project acording to this model, it will never work without exceptions and deviations.
Running a project is activly dealing with risks (the risk that a solution does not work as intended).
So the challenges for the project leader is to manage the risks and keep them under control.