They didn't manage it by hand, really. The system manages itself with speed droop flywheel governors. They were purely mechanical systems that made each generator on the grid 'push' a little harder when speed dropped, proportional to the generator capacity. Wince speed drops when load increases, the whole grid manages itself with a few springs and hydraulic valves on each machine.

Occasionally you might need to get a few more units online with fast load, transmission trips, etc but those events are infrequent enough to be well within the ability of humans to manage, especially with old trelatively small power networks.