which haruhi cosplay are you doing? presuming its the school uniform, its going to be similar to the cheerleading outfit.
the easiest method would be the find a pattern similar if not exactly alike the cosplays you're doing. for a cheerleading outfit(note: I dun remember what it looks like), I believe any pattern for a pleated skirt and a t shirt. patterns will have cut outs and instructions on how to put it all together.