Hi and welcome, Britney! I'm Olivia. =]
A short paragraph will usually do the trick! What I usually go for are the following;
-- Mentioning canon events and how they affected Brooke/ how she responded to them.
-- Talking to other drivers to work out connections with their characters, which you can then write into her history.
-- Any classes that give/gave her grief or that she found she had a knack for. Definitely mention this in First Year but you can revisit this as well for Third Year elective classes, O.W.L./N.E.W.T. choices, etc.
-- Any family events - siblings coming to/ graduating from Hogwarts, etc.
-- Clubs or activities that she joined or started.
I know I'm forgetting some, but those are usually the main ones I go with! Plotting with other drivers is one of my favorites because you can come up with things for multiple years with the same pair of characters. ^^ For instance, my Keela and TaylorC's Terry became friends early in school, then went through a period in Third/Fourth Year where Terry was jealous of Keela for making their House Quidditch team, which ultimately led to a huge argument (and resolution) this year and will end up dating haha.