What is the sash belt? Should I wear it?
The sash belt included with every order is optional. But it's particularly important if your shirt is fitting a little loosely through the chest area, which it can in certain combinations of shoulder width and bust size. You want your shirt to be snug through the chest because there's an important sensor in that area (even though it's hard to see), and if it's moving around too much you could have some problems with calibration or with your live avatar's accuracy.
So while optional it's generally a good idea to wear the belt, and it certainly never hurts.
To wear it correctly, wrap the sash just under the bra and tie it with the knot to the side. It should be snug, so snug that it's difficult to get a finger underneath it, but not so snug that it's uncomfortable.