Use suction cups to lift and set slab (as shown).
Press down firmly on hearth to adhere stone to thinset.
Check that hearth does not rock. If ti does, lift hearth and add more thinset.
If the hearth is meant to be flush with floor but is too high, tap on stone with the rubber mallet; if the hearth is too low, lift it and add thinset.
Fill joint beltween firebox and hearth with thinset. Scrape of excess and tool smooth.