Well, for the skirt, my friend and I just ended up cutting two pieces of almost rectangular fabric, sewed them together and attached elastic to keep it up (I personally would recommend an invisible zipper --> we had no time~). As for the sleeves, we cut two pieces of a trapazoid shape? (one for each arm) and sewed the sides together *leaving a little room on the bottom* and sewed on rectangular strips that can be tied into a bow...
I'm sorry if my advice makes no sense~
Here's a pic of my friend's Rikku to give you an idea of the result of the above...