You need a solvent to cut the ink. First try hairspray, it's an old trick that works well if you have nothing else handy. Next use denatured alcohol. Finally, if those don't do it, use aerosol brake parts cleaner. I have used all these methods with great success.
Soak a Q-Tip or medical swab with the solvent of choice (each one mentioned is progressively stronger) and lay the tip on the stain, push and twirl. Use several tips per area. Replace often. Then follow up with a rag. Treat the area afterwards with a leather dressing.
NOTE: it is always wise to test ANY leather for color fastness in an inconspicuous area first.