Start by preparing your icing. It should be ready to use for flooding before you add the black paste. Dip a small spoon in the paste colour and stir it carefully into the icing until you have a rough marble as seen on the photo. Be careful not to overmix, or you will lose
Prepare plenty of icing, more than you need. Use a shallow bowl for max surface area. As you work, the icing in the bowl will start to get overmixed (less and less marble) and you will want a nice big surface to scoop from.
Use a small spoon (this an old salt spoon) to carefully scoop the icing and drop it in place. Tap the spoon into the icing to spread it. If an area gets too dark, chose a lighter area in the bowl for your next spoonful.
Continue until you run out of area to flood and then you're done! Just let it dry.
NB: Large areas of unmixed colour can weaken your run-out, but since you will invariably be stirring everything around when you work, this is unlikely to be a problem.