It's really easy, it's just a rectangle whose ends are sewn together. If you google "infinity scarf tutorial" quite a few videos will come up.

With a thin, drapey knit, about half of a piece of 45" fabric, 2 yards long is good, but with a woven with less drape, you want either shorter or narrower. One of the tutorials uses a full piece of 45" fabric 2 yards long- don't do that, it's huge.

Sew the long side right sides together, turn it right side out, flip one end over twice, then match it to the other end and sew, leaving an opening of a few inches to get it turned right side out again. You then sew that shut by hand.