Is this the correct outfit?
I’m not sure which sash you’re referring to, so I’ll explain everything sash-like that I see.
For the gold-colored tie around her waist, this is a simple rectangle of fabric sewn into a tube, pressed flat, and tied around the waist. I would recommend a bias cut on the fabric so that it gets the same drape. You will want to cut your rectangle twice as wide as you want the width of the finished sash, plus an inch for seam allowance, and then fold it in half lengthwise and sew the short ends together, and the long ends together, leaving a gap of a few inches. You can then turn this right-side out, handsew the gap shut, and press.
For the green draped garment, this will require a bit more patterning. If you are able to create a pattern by draping fabric over yourself/a friend/a dressform, I would recommend that. The pattern appears to be comprised of long rectangles sewn together in the back, belted at the waist, and draped off the shoulders, but try experimenting with shapes to get the exact shape.
For the structure of this piece, you will want to add mid-weight interfacing to the top portion, and possibly some horsehair braid to the very top edge for it to stand outward like that. If you need it to stay up better, you can sew it to a garment or straps that hug your arms/shoulders (I would do a spandex inner sleeve) so that the top of the draped portion is able to stand away from your body without falling.
If it is something else you are having trouble with, let me know and I’ll try to find a solution. :]
I hope that helps! Good luck :]