Begin with a clear coat (or any very pale nude color) and paint the nail. Let dry.
At the tip, paint on the red in a triangular shape with the base of the triangle being at the side of the nail. Let dry.
Paint the green next, starting at the opposite side to where you started your red color. Paint so it starts wide and narrows as it moves across the tip. It should be very narrow at the far side from where you started.
Clean up the top edge by adding a stripe of white. Then add a clear top coat to seal it all.