Summarize this article:

No matter the substance that caused the stain, it’s crucial to wipe away whatever excess you can. For lighter stains, this might be sufficient. A nubuck cloth is specially designed for cleaning nubuck surfaces; nubuck cleaner is usually integrated in the cloth fibers. These types of stains are usually found on jacket collars and upholstery headrests. Leather degreaser typically comes in aerosol form. Spray it onto the stain, and let it stand for about an hour.  The leather degreaser will turn to a powder as it stands, soaking up the oil stain. Rub the powdery residue away with a sponge and leather cleaner. If the stain remains, repeat this process. It’s important to start cleaning the stain as quickly as possible before it sets, usually in the first 6 hours. Ink lifter is a greasy substance that typically comes in a tube, much like lip balm. Rub the substance over the ink stain until it is completely covered. Then use a nubuck cloth and leather cleaner to wipe away whatever remains of the stain. Brush as you dry the nubuck. This will keep any remaining stains from setting. Brushing the nap will remove any residue, keeping the surface clean.

Summary:
Start by wiping the stain with a nubuck cloth. Use leather degreaser and leather cleaner to loosen oily stains. Use an ink lifter for ink stains. Dry the nubuck with a blow dryer and brush with the nap.