Levelling garden for turf

Lennys_Shinpad

Hi,
I am trying to level my garden before putting turf down and could do with some advice on how best to do it.
The patch I am working on is down at the bottom of the garden. It was overgrown and the previous owners had spent years piling stuff down there. It therefore isn't very level.
I have managed to turn it all over with a space and have begun to rotavate it. I now need to try and get it broadly level before I order my turf.
Can anyone tell me the best way to get it level? I am assuming I need to use some sticks, string and a spirit level. The videos I have seen on Youtube all start from having a fairly even surface to begin with.
Here is a photo of the patch as it is today.
Thanks in advance for any help.