I began with about 15 Tyvek envelopes. These are Tyvek graded for this type of use. Mailing things that you don't want to get wet or damaged. Tyvek is also made in several other qualities an they make a version that is perfect for making bags out of. I wanted to try the envelope idea though.
I also sources several pieces from an old golf club carrier. Shoulder style straps, webbing, buckles. You could source from an old backpack of course. Or buy new stuff.
I also purchased about 15 $ worth of a heavily water resistant material from a suit maker in my neighborhood. You could use anything you want. I wanted a light rip stop material but bought something a bit different. The main purpose for this material was to help provide a bit more structural backing for the envelopes. I did some tests of just sewing envelope to envelope. and while it held up good to my abuse eventually the envelopes started to crease and small tears developed. Once sewn to the additional fabric the envelopes really hold up to even heavy wear and tear.
So here are as many images and pseudo steps that I took to make my bag. Without giving step by step instructions I think the photos explain for themselves. Its pretty clear that once I attached the first "cross" of envelopes to the black material that I purchased that begins the foundation. Near the end once all the pieces of webbing, netting, the shoulder straps were attached sewing up all the seams was all that was left. I wish I had some real process shots of me sewing up the last points. But clearly there were some rude tacticts done by me to get this bag into one piece. All the ugly sewing is only visible from the inside though.
Good luck and go make one of these. After a full year of use in New York City condition and cycling all over Im really proud to say its holding up great.