I want to go to an anime convention dressed up in cosplay but I've never done it before and I've only seen a few animes to give me inspiration. 
I want to dress as Haruko from Fooly Cooly but I don't like her whole outfit.  The costume I'm thinking of is a red dress but I always see people wearing a white long sleeved shirt under it in their cosplay pics.  I'd rather not wear that, so I was wondering if it's ok to modify the costumes a little bit. 
Sorry for the long message, like I said, I haven't dressed up in cosplay before and don't know if you strictly have to follow the costumes or if there is room for creativity.