Buy Soft fleece material about 1/2 yard of 90" width
I wanted to make one for my daughter...so i made it a bit shorter than I would for an adult. For an adult take the length you want plus add another 30" to make the ruffles.
My daughters finished neck warmer was 35" in full length. I cut a length of 56". The width which was 8" would be fine for both an adult too. For an adult this length measurement would be fine if you don't mind the ruffles ending just above the waist. I prefer it up to the hip or just above.
As each adult will be different in height...the length too would be different..So you could either take the maximum length you have and then once you join the edges of the length you could like drape it around you and cut off the excess ( which you can use for the bow) or like i said earlier ...add 30" to the length you want for the ruffles. If you want more ruffles...then more inches! obviously ! :-D
Once you have decided the length you want...cut a rectangle...In my case i cut 16"x 56"
fold it in half so you get 8"x 56".