I cut across a fluorescent tabard (given free at Skyride) rolled the cut edge and Mrs WJ sewed a pocket for the pole.
There is a no-sew option for flagmaking, too.
A hi-viz vest is a good starting point because they tend not to fray once cut, so hemming them isn't essential. For a triangular flag, cut a diamond-shaped piece that, once folded in half around a pole, gives the triangle shape you want – maybe 4 to 6 inches high and as long as you want, perhaps 12 inches. Put a strip of one-inch double sided tape down the fold line, and stick the pole – 3mm fibreglass rod works well – onto it. Then fold both sides of the flag around the pole to make your triangle and the double sided tape contacts itself. For belt and braces, two or three mini eyelets crimped through the flag and the tape will keep it there. Attaching the pole to the bike is an exercise left for the reader, but cable ties and bits of inner tube are useful.
The resulting flag, that I had on my Windcheetah at the time, was still going strong 15 years later with the new owner. I made a slightly more robust (less "flappy") version once by first hemming the edges, adding some of the Scotchlite tape I had previously unpicked from the hi-viz vest, and then sewing the two sides together. But I never used it because by then my needing-a-flag days were long gone.
