The "best" way is to double dig the entire plot, incorporating as much humousy material as you can get hold of, ie well rotted anomal manure, garden compost, bags of soil improver bought from garden centre, spent mushroom compost, etc, plus loads of horticultural grit.
The "easiest" way, is to do this for small areas as you plant, you can never have too much of any of the above, and eventually your soil will be perfect, because clay soil is more fertile than most, {remember that clay is only like sand with the rough edges rubbed off}, Derek.