Soft leather is not a cheap material, and the cleaning of it is can be difficult. Because of the sensitive nature of leather, cleaning methods used for other clothing or upholstery cannot be used on leather. Of course, you should always practice preventative care and avoid getting soft leather dirty, but if something has happened, you can clean the leather.
Gently rub a piece of 600 grit sandpaper over the soiled area of your soft leather item. Sanding will disturb the stain, and cause the dirt and leather to dust up.
Wipe the dust off the soft leather item with a soft, white cloth. If the area still appears stained, try scrubbing it gently with a stiff-bristled nylon brush. The bristles of the brush are able to get farther down into the fibers of the leather.
Clean off any debris with a soft cloth. If the stain won't come out by rubbing it with sandpaper or a nylon brush, it may be an oil-based stain. Apply enough cornstarch to the stained area to cover it completely, and allow it to sit overnight.
Examine the corn starch the next morning to see if it has yellowed. If it has, the corn starch has pulled oil from the stain. Brush the corn starch away with the nylon brush, and repeat the process, if needed. If the corn starch has not yellowed, the stain is not oil-based, and you should consult with a leather professional.