Working with what I had, I couldn't do it to the exact dimensions stated. I cut the back of the legs off the trousers, cut them to rectangles then sewed them together down one short edge to create one long piece approx 50" x 12". I then double folded the long edges so that not only the pom pom trim but also the fabric edge was caught inside before sewing.
I have worn it long and also doubled up and I love it!
I added a bit of decoration to the fabric by dipping the end of and old toilet roll tube in some bleach and stamping it on. Leave it for an hour or so to let the bleach work before washing.
I used 2m of pom pom trim which cost me £3 from a local haberdashery shop but everything else was free/recycled. £3 is pretty good for my new autumn scarf I think and its going to get a lot of wear.