Replacing / refastening fireplace hearth and back panel ?

Hello All ,
Can someone please advise me on the proper procedure for replacing a inset real flame type gas fireplace hearth and back panel (marble type )
Is the correct procedure as follows . 1. turn off gas to fire (disconnect as necessary) 2. remove the fire surround 3. remove from the wall old back panel 4. remove old hearth 5. make good surfaces ( one coat plaster etc ) clean and level hearth floor area .
The bits I am not sure about are as follows
what do you lay the hearth on (mortar I am supposing ) is it a full bed or just enough to bed the hearth level on ?, what is the mix 5 to 1 ?
how do you fasten the back panel to the wall ( liquid nails ? ) or some special adhesive ?
Do you have to remove all the fire from the wall or can you remove the old panel etc and just slot the new one into its position ?
Do you have to silicone in the fire back to the back panel ?
Thanks in advance for any help and advice .
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