In certain theatrical productions, costumiers want to distress their work to make it more appropriate to the show's setting. Some materials, such as leather, are more difficult to age or weather than others. Leather is a lot tougher than other fabrics, so costumiers have to use a different process when ageing leather garments.
Use the sandpaper to lightly scrape away the area you want to weather. Don't grind too deep into the leather, as this is intended to be the lighter part of the distressing process.
Use the steel wool to heavily distress the area you have just sanded. Start in the centre of the area you have sanded and work outward. This generally eliminates any odd shapes or irregularities in the final pattern.
Dye your garment. The dye colour should match the pre-distressed garment's colour.
Remove the garment from the dye after half of the recommended time. This ensures that the areas you distressed are close to the colour of the original garment but are still clearly worn and aged. If the colour is still too light, repeat step three until the desired effect is achieved.
Lightly mist the garment with black spray paint. This adds the illusion of dirt and grime, and also makes the ageing look more realistic. Be aware that some of this paint may come off during the laundering process.
This isn't an exact science. Experiment with different levels of distressing and how it affects the overall look of the garment.