Usually when soil is tilled or dug, the top 6-12 inches of soil is turned over. With double digging, you remove the top 12 inches of soil (or the depth of a garden spade), set it aside, and then loosen the next layer of soil. If you like, add and mix in several inches of organic matter as you turn over each layer. Top off the lower layer with the amended upper layer. Continue this process to the end of the planting bed. This is hard work; it helps if you have a strong back. You will notice a dramatic difference in the growth of plants in the double-dug beds, however. When roots have 2 feet of unimpeded growth, plants thrive. You can get some of the same benefits with less digging by building raised beds.