Ginny already had her person, Slytherin student Millicent Bulstrode. A fairly unremarkable witch that wouldn't be missed so she was a good target, and also the strange case of a girl in a wizard school that behaved like a muggle bully, using her physical advantage to intimindate others. Ginny despised that kind of behavior, and knew the only reason she could get away with it was ironically, because attacking back with magic and putting her in place was something that was frowned upon by the professors, while apparently grabbing another student in a headlock, or pushing them or throwing the books they were carrying was "just a phase".

But she needed the right time and the right place. It wasn't like she just could tell her drink the potion she made! 

While Ginny was pretty confident about the effects of the solution, she wasn't sure about how they manifested specifically. She knew the potion would be able to shrink its target, but the minute variations when preparing it could influence a lot of the result. Would Millicent shrink instantly? how much would she shrink? Those were questions that Ginny could only learn to answer with practical uses of the potion, so for now they remained unanswered and she needed to be cautious.

It took her a while to prepare a plan because Millicent wasn't exactly a proactive bully, she needed motivation or orders from others to act, but eventually she had a working idea.

"Come here Weasley, give me that!"

"It was a gift!" Ginny cried as she ran towards a private spot, potion in hand. She pretended she got a concentrated felix felicis potion and proudly announced it around as something that would surely make her a very powerful witch because of the special way it was fabricated. Most would be righfully skeptical because things in the magic world just don't work that way, but for a mediocre witch like Millicent it did the trick and when Ginny started to leave the Great Hall, Millicent chased her.

"Ack!"

Ginny was athletic enough to outrun the bully but that wasn't the plan, so she pretended to fall down and let the vial escape from her grasp, now that nobody was watching. Millicent caught her and put her shoe on Ginny's back.

"You should leave those things for people that deserve it Weasley" the bully laughed

Ginny growled but internally was excited and hoping the effect of the potion was instant, she already had ideas for the payback!

Millicent wasted no time and drank the potion.

What happens next?