There is an EASY pattern for ponchoes, we used to make as teenagers many moons ago…
you take 2 rectangles [ lets say 15 inches x 30 inches — that sounds reasonable ].
then you sew one edge [ 15 inches ] on the edge of the second rectangle 30… now you have an L shape.. now take the 15 inch edge from the second triangle and sew it on the 30 inch edge of the first. if you take 2 pieces of fabric for an experiment, and figure it out , you’ll see how easy it is.
all you have to do is sew/ crochet/ knit rectangles and sew 2 seams . you could even decorate the seams and let your imagination go wild.
the width of the rectangle is the length from your shoulder to the wrist [ or longer if you want ]. you will just have to work on the length of the rectangle [ how long on that ]
this is sooo absolutely simple. and you can knit, crochet or sew these rectangles any way you choose.