I buy brassica plants and just throw them in by digging a quick hole. Then they get a very good watering. But I do use plenty of fertiliser. I use both blood, fish and bone and chicken pellets and a mulch of manure or compost if possible.
I do not think Brassicas like dug soil so for me no preparation is needed. I planted some where I had had strawberries and simply scraped off the surface. Ground was hard so had to water to even dig the holes. They had a sprinkle of BFB down the hole and a sprinkle round the plant and then the pellets as well. They are doing fine. I also put some lawn clipping round since it deters the slugs.
So I would say forget the preparation but feed, feed, feed.
And not forgetting the nets because the pigeons just love a nice juicy brassica.
