I Found this how to link at Velorution.

Via: 5

last summer L. and i wanted to steal one of those orange banners that announced the minimal future exhibition at the MOCA with a richard serra piece to make a bag out of it a la freitag. imagine that, a richard serra bag. we even profiled a few lightposts that we could climb to get the material but never did it. L. bough some vinyl downtown and we did some things with it and decided that we liked the material so the idea hanged on. a few weeks ago i was riding home from work and after stopping to check two bicycle shop dumpsters (you know, they thrash all kinds of usable stuff) i went by certain banner printing place to check if they were throwing any banners away and surprise! their dumpsters were full of material. there were people working that day and the shops were open to the alley but i grabbed a big piece of vinyl and rolled away in my skinny bike. i must have looked like a cat grabbing a big chunk of cheese. one block away i folded the thing and strapped it to my bag. so it was time to make a messenger bag.

Check out the full how to here. They have plenty of pictures and blue prints.