How to double dig
Digging is best done in the autumn as it gives time for the soil to be broken down by frost. If you dig at other times and your soil is not fine enough for seed sowing you can add a layer of home made compost, or commercial peat free compost, over the surface to make a good seed bed. Heavy rain compacts soil so if you do dig in the autumn cover the bed to keep the rain off.
There is still some debate about the need to double dig every year. Digging is said to destroy soil micro fauna and to encourage the germination of weed seeds. The no-dig approach certainly work but caring for soil is much more than just not digging it; you will certainly have to use various ways to avoid soil compaction including; not walking on the soil and keeping off heavy rain in winter.
Here is an introduction to double digging. You will almost certainly have to do it at least once when you make a new bed. (Click image to enlarge)