Blot (don't rub) the grease stain gently as soon as possible after it occurs. Plan to either treat or professionally clean it as soon as possible for the best results.
Make a mixture of 1 part cornstarch and 1 part talcum powder. Sprinkle it on the grease spot and allow it to sit overnight.
Gently brush the previous mixture off the grease spot and reapply. If the mixture is effectively working to absorb the grease, the stain should be faded.
Repeat steps two and three until the stain is gone or no longer fading.
Gently brush the spot with an art gun or a very soft eraser.
If, after these steps, the grease stain is still there, you should take the item to a professional leather cleaner.