You will need to go down about an inch then sand then the artificial grass. You will need to sand the top too, you should use a 2x2 batten around the edge to fix you’re grass to

After removing the turf you should remove 100mm of soil, then spray with weed killer. Lay a weed membrane then 100mm of sharp sand levelled and compacted. You can get large metal staples to hold the lawn in place.

Depending on the thickness of the artificial grass deters how far to dig down and excavate the area you want. And also to decide if the area would need retaining with a timber or brick boarder etc.

For example if its 40mm ply artificial grass, you would need to allow 50mm for a sub base of preferably millwaste or hardcore but applying a secure layer of weed suppressant membrane first before spreading the millwaste which should be compacted level, then apply a good 20mm of compacted and level screened sand. Then leave another 30mm for the artificial grass. So in total a 100mm dig off but leaving 10mm of the full ply of the artificial grass so it stands proud in the area.
You can apply kiln sand for aesthetical purposes but it's not essential.
And if you where to have a boarder this would be installed first with the artificial grass laid after to give you a level and edged perimeter to work to when laying the grass and cutting in if needs be.

Dig area down 100mm Remove lay type 1level compact screed sharp sand then lay artificial grass