What preparation was needed for d-day landing 1944 to take place?

Exercise Tiger is a good example where nearly a thousand US GIs were lost in preparation for D-Day when a convoy was ambushed by German E boats off the Devon coast. A good book by Ken Small, The Forgotten Dead, tells of the recovery of a Sherman tank from the sea bed which is now a war memorial on Slapton Sands, South Devon. Another feature was the disinformation of FUSAG, the mythical First US Army Group, nominally commanded by Patton, which didn't exist.