Well most good Nabooru cosplays ignore the giant nose... Her ears are also just like a human's, so don't worry about those!
When I made the gem I was low budget and on a deadline so I used craft foam. I cut it out in polygons and then used hot glue to put it into the shape of her gem. I then sewed that into the wig we used. If you want to do anything more complex then you're looking into spending a lot of time and money on casting the gem/etc... For the other jewelry on the costume I suggest clay, beads, wiring, or other conventional jewelry making options.
The pants are essentially "harem pants". Many patterns and tutorials can be found online. The shirt is often done with just a tube top.