Draw or trace a simple drawing of the pattern with which you want to decorate your whirligig onto a piece of paper.
Tape the paper to the metal surface on the whirligig that you want to decorate. Lay the metal onto a surface covered with a layer of cardboard.
Line the point of the nail up with a point on the drawing, then tap the head of the nail gently with the hammer to dent the metal.
Move the point of the nail 1/4 inch along the drawing and make another dent in the metal. Continue tapping dents along the lines until you have traced out the whole design.