depending on thr area-would be best to rotovate
or spray to kill grass first
or burn grass (large gas blowtorch)
or take off the top sods of grass and dig up the under soil then add some decent topsoil and dig all in together
- just hand digging a large area would be a big task unless you want to do it that way
It is best not to dig the turfs in. Take the turfs and find a spot to stack them grass down and leave them for a couple of years. Then you can use the soil on your garden.