I did one on direwolf years ago, not sure I can retrieve it in useable form. This was for banners for 28mm fantasy knights (Warhammer Fantasy Battle Bretonnians).
Basically you produce the banner both sides on paper. (More below about this part.) Use successive coats of thinned white glue as a sealer and stiffener and ultimately as an adhesive and sculpture softener. Once the banner artwork is ready, cut out and wrap around the banner pole, sticking it together with thinned white glue. Let dry. Trim. Paint the edges black or another dark color appropriate to the color scheme.
Paper: Plain paper such as copier paper or printer paper works fine. In the beginning I also used Post-It paper a few times, but it is thinner paper and turned out a little more delicate.
Sculpting: To shape as if blowing in the wind, soften with thinned white glue, shape, let dry, repeat if necessary.
Producing the banner artwork: You can of course print out any artwork you can find online. For printed artwork, reproduce by scanning or photocopying. If the image is one-sided, scan into a graphics program and make a mirror image to create the other side, or do the same via photocopy. If you want to hand draw one, do it 2-4 times bigger in big bold cartoony detail then reduce via photocopier or by scanning and printing scaled down.
(I can't emphasize this last enough. In the YouTube tutorial posted above the banner is hand-drawn in the finished scale size. This takes a fine eye and steady hand, especially in realistic genres. It's much easier to generate good-looking intricate detail if you draw it oversize then reduce it. Note professional graphics artists and cartoonists typically create oversized original art for the same reason.)
Colors: Easiest is to make a black and white banner then hand paint it with thinned paints. (Before painting, paint with thinned white glue and let dry to stiffen the paper and partly seal against absorption. If the paper gets soft as you paint it, let dry then come back.) If you have access to a color printer you can print color banners, but it's hard to match your paint colors exactly so print darker than you want then overpaint.
Colorfastness: The inkjet inks I used didn't run when dry but YMMV. Seal with thinned white glue to be sure.
Markers: I did use colored permanent markers a few times. They work fine except beware later when making quick repairs. Superglue makes the ink run.
