Query about the slab plastering

From what I have understood from your question, I am assuming that you are asking about finishing the roof of your house. 

While there are various methods to finish the roof depending upon one's taste and budget, I will be talking about the most common medium i.e. brick tiles for roof.

After applying a proper water-proofing coat over the RCC slab, cover the slab with mud with proper sloping into rain water pipe(s), lay 1" thick layer of wet mud mixed with bhusa/phuska/tudi and fix brick tiles over this layer with wet mud. Finally use cement+sand mix to teep the groves of these tiles. 

I hope it helps.