Usually, it's a matter of removing the grass and the topsoil.
This means excavating to around 300mm (1'), then filling with hardcore.
The idea is to remove all "deleterious" material. A fancy way of saying that there should be nothing left from which weeds etc could sprout up.
Tends to be a tad over the top in cases like yours. Dig out to 4" and fill with 20mm "scalpings" (20mm chips going down to dust, with all sizes in between.) Can be obtained in 650kg dumpy bags (craned off)
OR... standard concrete mix of chips and sand, (without the cement.)
Something like that.
Spread an inch or so of sand (much the same as for the driveway pavers we talked about.) No real need for plate compacting since it's only pedestrian use.
Set the slabs down after you've levelled it all up.
If you're worried about weeds, then lay some ordinary polythene down under the scalpings first.
;o)