Last year I laid out cardboard to clear the underlying grass and weeds. This year I tilled that up and flattened the earth, added about 1.5" (3.81 cm) of levelling sand and tamped that down to compact it. On top of the sand I did a layer of pine bark mulch to doubly prevent weeds and promote moss growth between the stones (moss likes acidic surfaces). I think I’ll put some more mulch and sand down and sweep it into the cracks when I have the rocks all fitted together a little tighter.