Protect the area around the staircase. Place and tape plastic drop cloths around the stairs. Do not allow any foot traffic on the stairs until you have finished the project. This will prevent potential slips on the cloths.
Sand down any rusty spots. If you notice any rust or metal spurs, these will need to be sanded down, using steel wool or a sandpaper graded for use on metal.
Prime the wrought iron. Use an oil-based primer that is intended for use on metal. This primer will help seal the metal and protect it from developing rust in the future. In addition, you will get a truer color application from your paint by using primer. Use a small brush and sweep the paint on the iron. Be careful with detailed areas or curly-q's in the metal; these will often drip. Smooth any drips out as they occur. Let the primer dry for at least four hours.
Paint the staircase. Once again, use an oil-based product that is made for use on metal. Failing to do so will result in peeling and chipping paint. Use the same technique for painting as you did with the primer. Work slowly and put an even coat on. Let this dry for four hours and examine it. If you need another coat, apply a second light coat and let that dry thoroughly.