Select one short side of your garden to start on. Dig down this side of the garden to form a trench 8 or 9 inches wide and deep. Place the displaced soil in a wheelbarrow or on a spread out tarp. Consider this trench to be your first row.
Dig a trench parallel to and right next to the first trench or row. With each shovel full, lift the soil and flip it over into row one. This will place the deepest portion of soil from row two at the surface of row one.
Break apart each shovel full of soil after you dump it into the previous row by stabbing at it with the blade of your shovel. Remove any rocks, roots, twigs or other debris as you see it.
Continue digging new rows or trenches, one at a time, by lifting and flipping the soil over into the previous row until you have dug the last possible trench and it rests empty. Move the soil from your tarp or wheelbarrow to the last row, and dump it into the trench to fill it.
Repeat Steps 1 through 4, this time working along the longest side from one end of the garden to the other to further lift and mix the soil.