Start by taking the tapered piece of wood in one hand and your knife in the other. Why would I say that? You always start out that way. Hold the piece of wood against your bench, at what will be the top of the spiral, once it's whittled. Lift the piece of wood up a bit so that the smaller tapered end still rests on your bench. Position your knife at an angle against what will be the top of the spiral (where you want the spiral to start). Apply a little pressure to the knife and begin to twist/turn the piece of wood into the knife blade. Hold the knife steady and still as you turn and slightly "pull" the piece of wood towards you. The knife blade will make a spiral stop cut up the piece of wood. You can vary the "size" of the spiral by varying the angle of the knife blade as well as the "speed" at which you pull the piece of wood into the knife blade.
Once you have the spiral stop cut completed. merely remove small chips of wood down and up into the stop cut.