Start around the outside of the garden and work your way toward the middle, cutting it into strips.
I recommend using a flat-bladed spade (mine has a short, D-shaped handle which makes the job easier) to cut the sod into strips that are only slightly wider than the shovel’s blade. Start around the outside of the garden and work your way toward the middle, cutting it into strips. You don’t have to cut down deep; maybe about 3 inches.