You won't have to pick any stones out that are smaller than 75mm but the soil will need to be worked if it's not flat then the lawn won't be flat and you could have loads of highs that the lawn mower would scalp or lows where the lawn mower wouldn't cut the grass properly. Plus aesthetically it would look much much better being flat and all in the same plain.

Hi, it's always best to start with the ground as clear as you can get it, get as much of the stones off as you can and if the ground is compact and hard then rotavate it so the fresh grass has a better surface to root into and establish itself, also yeah get the area you want to turf as flat as you can, mainly just so it looks nice, also the better you can level the area the easier it will be to maintain as time goes on. All the best, Matt.

You should have no stones then flat it with a vibrating plate then a light rake so the new turf will take

当铺设lawn we tend to rake as level as we can and remove as many stones regardless of size. When happy we would build up soil with new top soil grit mix and rake till happy with level ,then lay the turf.