The best way to remove pen marks from your leather is to let the mark dry and then take a damp cloth and apply it to the area.
Rub gently in a circular motion until the pen mark wears off. You may have to repeat this process a few times on the more stubborn marks.
Be sure not to get your leather too wet, as water can damage some types of leather.
As you know, leather is skin and skin absorbs things like water very quickly and takes a while to dry off.
You do not want to cause any unnecessary damage or wear on your leather. You might want to call a shoe or leather repair shop for more information on how to treat the type of leather you have.
So this is how to remove pen marks from leather.