Here are some etsy sellers that make them. Check 'em out for inspiration. Red, Blue and White. Stripes and solids. Pumpkin and Brown.
First find a couple old tee shirts. Thrift stores are best for finding cheap tees. Also, and this is important, you want a knit that will curl well on the sides when cut. I've found that knits made mostly of rayon or lycra work best. The thick 100% cotton kind doesn't curl and just look like strips of fabric when you're done. I recommend tee shirts instead of loose fabric because it comes pre sewn on the sides. But if you must use loose knit just remember to sew it into a loop.
Although I think these look cool. I have to say, they aren't that warm. Unless you wrap them tight around your neck but I think most people wear them loosely. They might keep the back of your neck warm but for me, it's the front that needs the protection. If that's a problem for you too, I recommend making a little one that ties in the back, like this one below from etsy seller elenorashop! It might take a few more steps but it's still quite easy.
So here we go....
1. Cut top of tee off just under the armpits. If you're making several of these, to make things go faster you can layer each tee on top of one another.
2. Fold fabric so that the two seams almost meet. Leave an inch of space between the seams.
3. Cut from the bottom up into strips. Leave the top seam uncut.
4. Lift up fabric by the middle (where the uncut seam is).
5. Hold by the middle and start gathering the strips.
6. Like so.
7. Stretch out your fabric.
8. Cut one of the connected seams lengthwise along each strip.
9. Cut off one strip from the scarf. Using that strip make a knot around the remaining seam.
10. Wrap the strip around the seam to cover it completely.
11. Knot it at the end around a strip.