To use the contrapotion you just need to click to the green potion icon at the top left of your screen next to the chest icon.
To become a succubus you need to put the cum tank top that annie give to you and expore tale street to get some cum (you will need some glass vial too for this) Take 3 cum vial and come back to Annie, she will explain you the continuation of the quest, you just need to follow it to become a succbus.
Btw how do you know you can become a succubus ? Is there a quest about this that I missed?