You would first need to dig out the area where the slabs are going to go. Generally if just being walked on you would need a base of 75mm mot for the sub base, this is so that you have a solid foundation for the patio to sit on.
Then a mix of sharp sand and cement at a ratio of 4:1 with some water just enough so that the mix binds together, the depth of this is generally around 30mm.
Then you have the thickness of the slabs again generally around 20-50mm.
so you would need to dig out around 125-150mm/5 or 6 inches to start, then lay down mot level this and compact so its nice and firm.
Then mix your sand and cement lay this and level on the mot, then your slab goes on this and tap level using a rubber mallet and level so that all the slabs are even.
I hope this helps.