Right first of all, you need to make sure the sink is free from loose dirt and grease. Wash it down with bleach or vinegar.
Next you paint the sink with a masonry glue (Wickes sell this in small tubs)
So now for the covering. Mix 2 parts of sifted peat or compost bark or a reasonable quality peat free compost with one part sharp sand and one part cement. Add some of the glue to the water when you mix up the hyper-tufa (That is what this is called. Don't make the mix too sloppy, but it needs to be wet enough to slap on the sink and stick. Wear gloves for this Concrete is a strong alkali and can burn skin. Spread a good inch or so thick layer over the sink and cover it with damp cloth and let it dry out slowly.

If you have to time and energy you can spend a few hours scratching off the glazing from the sink, but I have never bothered and the hypertufa stays on as long as it needs to.