Watashi wa S ni Sakaraenai
From Decadence:
Miku, a poor young girl, becomes the mistress of her father's creditor to pay his debt. This bigshot boss is known for his numerous conquests and his inclination for "professional" young ladies. Despite her good intentions, Miku has a big handicap: She has no experience. To her distress, the butler of the mansion decides to help her by taking her formation upon himself.