Cover all areas with a heavy-duty exterior primer. This can be purchased from a local hardware store. Oil-based paint would be the best in this situation, as the consistency creates a protective layer on the metal surface. Watch out for openwork designs in the metal that can result in uneven application of primer.
After the primer dries, apply a coat of an exterior paint. This will be background color for your herbal designs. Choosing a sky blue or white would be good as it will contrast the earthy colors of the herb.
Use a pen to mark out where you want to stencil on the images of your herb. Make a small mark on the metal that will easily by hidden by paint. Repeat this as many times as you want to indicate where you will stencil the print. The goal is to plan the design before you paint.
Line up the stencil according to your markings and use a solid green exterior paint to outline or fill in the shape . Make sure the paint dries on the first print before continuing to use the stencil for consecutive prints. Especially if the prints are close together, be careful with painted edges. They can take longer to dry and easy to smudge.
Accent the green images with contrasting colors. This can be done freehand if you desire extra detail on your furniture. Small strokes near the edge or thin lines characteristic of a leaf throughout each image can be good. Use earthy colors like lighter or darker greens, browns, and tan. The specifics of this step will depend on how complex you want to make the prints on your furniture. If you are unsure as to the level of detail you wish to include in this project, stick to simple green prints with some shadowing effect around the print.