Once you are happy with the site and shape, peg out string to mark the edges and, using a nice sharp spade, remove the existing topsoil and turf to a depth of about three inches. If you are converting lawn, I don’t advise turning the turf over and digging it back in. You will be digging out grass for years to come.
Instead, recycle your turf in your compost heap.
Now for the hardest part. Use a garden fork and turn the soil over, breaking up larger lumps and adding in well-rotted organic material and new soil or compost. For bigger patches use a rotavator. You can hire them by the day and they will save you some of the most backbreaking work you will ever do.