Hermione jumps up eagerly and leads you to the aforementioned girls bathroom (Ron looked visibly uncomfortable). Her cauldron stands right there in the middle of the aisle, when met with quizzical looks she explains that this is one bathroom everybody avoids going in to, because it's the one haunted by Myrtle. Thankfully, she seems to be absent at the time.
Hermione sits down and adds a couple more drops of some thick black material to the cauldron. The contents of the cauldron bubble up almost to the rim (you take a step back just to be safe, you've been burnt one too many times before) but the liquid settles back down, changing the colour from vivid green to more translucent shade. 'Oh good' sighs Hermione.
According to her, all you guys needed now are a few more herbs that should be available at the Potions class cupboard, a piece of whomever you want to turn into ('Come again?' Ron exclaims in disgust) and to keep stirring this continuously for another hour.
If you want to do this tonight, you better split up. It's up to you which task you'd rather take on!