5 of plastering sand, 1 of cement (white or grey), 1 of lime. You wont need very much, so you can use any small container as a measure. Add a little waterproofer/plasticiser to the water, stir well, and add it sparingly to your mix. You wont need much water if you mix the mortar briskly, and keep turning it over with a shovel,,, it aerates the mix making it light and buttery to work with.
Roughcaster.