I made one today out of a cashmere pashmina that my mother had brought for me from Isreal. I did as someone else mentioned and folded it lenghwise, and sewed up the seam on one side only, leaving about 12 inches or so, from the fold, for the neck opening. I wore it out today and it worked really well!
I wore it in fall weather over a cashmere crew neck sweater. The "poncho"has more something of a boatneck, and an assymetrical hem, which I loved! You need to use a scarf that is a weight you like. I have some cotton ones that would have been great in the summer, but I only got this brilliant idea this morning. LOL
I didnt even lock the sewing seam, so if I want to I can easily unpick the sewing and get my scarf back. (I have the feeling it's going to be living it's life as a poncho now though. I have plenty of other scarves!)
Try it, you have nothing to lose.