a quick pass through for the swimsuit quest, i didn't confirm it because i really don't wanna go through it again:
do whatever until the step where you have to steal the suits,
steal their favourite suit on the corresponding days,
monday - luna
tuesday - daphine
saturday - susan
sunday - hermione
check the notes section in your journal
give the suits to the sorting hat, then generate and replace all their suits with hermione's pattern(speedo?) before sunday ends.
to replace a suit you visit a girl's dorm and simply grab any random piece of clothing from the room's clothesline and replace them with the corresponding girl's speedo
days might be wrong but the general idea is to replace everyone's favourite suit with hermione's pattern.
what i did was stealing all the suits during weekdays, then replacing the suits during sunday.
after that talk to sally, then invite snape over for a drink, talk about new rules, respond with he's had too much of a drink. after that you just tell the girls about the changing of the new rules individually, then the quest should be marked as cleared.
edit : the linked walkthrough video someone posted in this thread also shows that you can exploit the phoenix event to steal swuimsuit off hermione. i dont know if it counts towards stealing her suit though