In the rough route (refusing to help Iris): if Iris removed the jar, talk to your mistress the following day while not wearing underwear. If she didn't, try and tell on her to your mistress.
In the gentle route: talk to your mistress the morning she leaves while not wearing underwear.
That will get you the diaper. after that just wear it for every scene. Lily also has some special diaper scenes on her dates. I recommend picking the pink diaper for extra content.