Mortar Mix for Cut-Down Sandstone Coping Stones

[Outhouse18]

I am wanting to use cut-down sandstone roof blocks as coping stones to cap the walls that used to be covered by the roof on an old outhouse. The walls are double skin with a cavity. The inner walls are all brick, some of the outer walls are stone and some brick.
I've never mixed mortar before and want to get this right.
Can anyone advise as to the best ratio of sand to cement? Should I add lime, plasticiser or anything else? I will be mixing enough mortar for only one capstone at a time. The stones will be 400mm x 300mm and 65mm deep.
How deep should I make the mortar bed?