Hey, this is cute! I probably have enough in scraps to do this ( hahaha )! I haven't watched it yet but I love that she has some fun colour. What exactly do you need help with? It's a pretty simplistic costume... if you can't eyeball it, you could probably buy a cheap pattern for a toga and do some slight adjusting. It's basically just rags though soo... I wouldn't even go as far as spending money on a pattern.
As far as materials, again, due to the nature of the outfit I'd stick with something relatively inexpensive. A plain cotton or knit would do just fine. I can't tell if that's a pattern or just crinkle on the light aqua, but you could dress it up with some embroidered cotton on the trim instead of just layering basics.
As for the leather belt thing, not sure since I haven't seen it all the way around.