I need to fit a marble hearth. Well it's not actually true marble but one of
those conglomerate jobbies.
It's about 54" wide and about 14" deep (front to back) and 1/2" thick
(approx.). This sits on a frame composed of the same material along three
sides (left, right, and front).
So, I'm looking for suggestions as to best method of fixing to a concrete
floor.

Should I just stick the frame bits onto a mortar bed (what ratio of
ingredients would make the best mortar, or can you buy ready mixed stuff?).
I thought I'd fill in the inside of this frame a little too. And then when I
fit the top, I thought a few blobs of mortar would give enough support. But
what to use between the top of the frame and the hearth itself?

The previous hearth was one damn heavy solid slab which sat on yellowish
coloured mortar(?). Nearly lifted it in one piece too.
Thanks for any comments.

Use plasterboard bonding for the floor and sides to wall ,and a no nails or
similar for any fixing of tops or sides.

Alex wrote:
Use plasterboard bonding for the floor and sides to wall ,and a no
nails or similar for any fixing of tops or sides.

Thanks. Sorry for the late follow up (by me).
Why would you suggest plasterboard adhesive over a regular mortar? Also,
what about floor tile adhesive?