This is just a simple "flop over" method for wearing Crim's ascot. Normally there would be an additional wrap before the flop, which works really well for flimsy materials like silk but not so much on velveteen. If you're using one of my Ascots you'll notice the seam is not perfectly centered. When you put the seam at the back of your neck one side will hang lower in front - that's the side you'll wrap and flop.
Just cross the long side over the short side, come up under the neck, flop it over and adjust. You could also secure it with a tie tack if you like, but the velveteen isn't going to slip around like a silk tie would.