Tools needed:
If you can find an able-bodied person with a strong back and a weak mind, talk them into doing this for a portion of the harvest. Originating in France during the late 1800's, double digging is a labor intensive method of cultivation that aerates the soil to a greater depth than conventional tillage and provides excellent drainage as well as a way to incorporate organic matter at a greater depth than any other method that I know of. One of the advantages to this method is that there is less need to concern oneself with having the organic matter thoroughly composted, since the plants roots will not be placed in direct contact with it. They can go after it as it becomes ready. It is also an excellent way to prepare soil for raised beds. Chiropractors love it as well.
We will begin by marking the area that will be double dug. I would recommend laying out a fairly small bed first and tackling a larger one once you get the hang of it (and know what you're getting into).
Let's say for the sake of illustration that we will double dig an area four feet wide by twenty feet long. You can mark this with a stretched string, if you like, but I prefer to mark the boundary with a little garden lime. It doesn't get in the way like the string and is garden-friendly.
Now, grab your shovel. I prefer a round-point shovel, but a spade shovel may be your preference. Dig a trench as deep as the working end of the shovel and twice as wide, across the bed or four feet long. Carry each shovelful of soil to the far end of the bed and place it just outside the line marking the boundary. Don't worry, you only have to do this once. We will come back to this ridge of garden soil later. Now is the time to use the wheelbarrow to bring in compost, manure, biochar or what have you, as long as it is organic matter and spread it in the bottom of the trench you just made. One or two inches will be fine. Now take the digging fork and loosen the soil in the bottom of the trench. This should give us a total depth of about twelve inches or so.
Now dig another trench across the bed just like the last one and turn each shovelful into the trench we just finished digging, filling it back up. If there is sod, place it grass side down. Add compost and loosen the soil in the bottom of this trench with the digging fork and keep repeating this process until you have reached the other end of the bed. Now you should have a trench with a pile of soil next to it. Put the pile of soil in the trench and take a well-deserved break (even if you have already taken several...go ahead, you have my permission!)
Now your double dug garden bed is ready to plant and will be a great place for such crops as asparagus, potatoes or most anything which prefers a deep, well drained garden loam
You will be happy to know that it is not necessary, on most soils, to double dig the same plot every year. One double digging can provide benefits for many years, depending on soil type and crops grown.