Tumble and beat the jacket to soften the leather and give it a generally worn appearance and feel. If the jacket is new, start by running it through a few dryer cycles on low heat with a tennis ball to soften the leather and dull the original finish.
Hang the jacket from a clothes line and hit it repeatedly with a bat or stick. You can also place it in a plastic bag and run it over with a car several times. Try to age the jacket evenly, instead of confining it to specific areas.
Pull on the leather to stretch it slightly, particularly around the elbows and on the front. Stretch single pieces of leather, rather than multiple pieces stitched together, so that you won't tear the seams.
Start by pulling gently, as you only want the leather to be slightly stretched. Some leathers will stretch more easily than others.
Use sandpaper or a wire brush to scuff and scratch the leather. Use only gentle pressure to avoid obvious stroke marks, and make sure the scratches do not all run in one direction.
Pay extra attention to high-wear areas of the jacket such as the ends of the sleeves, the bottom edge, and the elbows. To make it look more realistic, concentrate the scuffs and scratches in those areas most prone to wear.
Make sure to get rid of any residual gloss on the material, though. To take the distressed look a step further, use firmer pressure on the edges to create small tears in the leather.