I am unsure what you mean so Ill anserw few possible questions:
-How do you get the potions?- explore till you find witch Lady(Lucia I think) selling special items, if you don't see potions she changes inventory of her shop every so often. Buy potions and then open inventory to use them. Gina will learn that all but 2 potions are pass expiration date and drinks one, leaving you with the one for Stella
-How do I gove it to Stella - you see how when you click train there are options to train with others. If you meet requirments(idk which ones but I think training few times with Jade and Stella should be enough) you will get an option with "Story" written on it. Click it and you'll train with the two of them and Stella will get angry at her lack of results. She leaves, you then have to go to her brothel to speak with her and give that potion if you have it