A womans sash is not much more than a quite long scarf you might wear in winter. A mans' fly plaid, which is supposed to resemble the top half of the much older style kilt, is wider and longer, and most often has an attachment device on one end, to hold it to the belt. Sometimes they are fringed on one end, even al four edges.
The construction of fly plaids may vary, but the one I have is just a rectangular piece of tartan cloth, fringed on all edges. One corner of it is pleated with two simple stitches, and meant to act as the corner that gets pinned to the jacket. Basically, the small tucked pleats give it more strength at the pin point, and make for a more decorative appearance. There is no attachment device, though. You would simply pin the brooch (or whatever type pin you're using) through the fabric. And a fly plaid is not meant to attach to a belt. It simply drapes over the shoulder, with the only attachment being the pin at the shoulder.

But again, this may vary between makers. I'm not really even sure there's an industry standard on how they're supposed to be made, since they don't have much of a history in Highland tradition.