This is a rough-draft recreation of the Achaemenid sash and weapon belt.
The sash is a length of plain-woven cotton belting or "web," which is not strictly accurate - it should probably be wool (as cotton had to be imported from India and was quite expensive), and the only evidence for the type of weave I have found, the Egyptian statue of Darius the Great, has a fancy pattern which I would surmise to be herringbone twill with additional strips appliqued with a holbein stitch along the entire length of the sash. Whatever material you use, it should be flexible enough to pleat and knot easily, so not too heavyweight.
When worn with a weapon belt, the sash and belt appear to be about the same width, which looks to me somewhat more than an inch, perhaps one and a quarter to one and a half inches (3.2-3.8cm).
First, square-knot the sash around your waist and cut it to the proper length. A good length for the hanging ends is about six to 10 inches (15-25cm).
Next, double-whiptstitch the cut ends to prevent them from unraveling.
My technique for creating the little round things on the ends may not be accurate, but it's easy enough:
Fold the end into pleats.
Draw the needle and most of the thread through the pleats.
Wind the thread tightly and heavily around the end and tie it off several times before trimming off the excess.
Interestingly, a shorter and narrower, but fundamentally similar, sash is worn by a Parthian on the Arch of Septimus Severus, seven hundred years later.
The weapon belt is made of heavyweight, vegetable-tanned leather - about 8 ounces is a good weight. Wrap it around your waist to a comfortable snugness and mark with a pencil where it should end and where to punch the holes. A few inches of overlap is good. Make holes with a leather punch, awl, drill, etc. and insert the leather lace.
Nothing quite resembling the conical bronze conchos used by the Medes and Persians is currently being produced by anyone that I've found, although a jewelry-casting studio should be able to make them easily enough and I intend to look into that when I'm not so unemployed.
As a stopgap measure, here I am using a "northern-style" concho, which is widely available from Native American craft suppliers. It's made from heavy brass sheet, domed, and usually sold with a soldered loop on the back. Soldering isn't a rock-solid method of attachment, but don't worry; the way we're using it here doesn't require the solder to bear any weight. The belt and concho in the above pictures are 1-1/2 inch and 1-1/4 inch, respectively.
Finally, position your bowcase and/or dagger scabbard just forward of your right and left hips, and punch holes in the belt to attach them.
For comparison, see page 5 of Nirupars' Persepolis gallery, second row from the bottom, middle photo.