Here is the complete set panniers I made for my touring bicycle. I simply expanded on existing ideas I found on Instructables and other sites concerning the fabrication of homemade panniers.
I had already started coating the bags. The material is a polyester canvas. I welded the seems together with a flat tipped soldering iron and put zig-zag stitches to add strength. I just cut a long piece of material and made a squared U out of it. I then used two rectangle pieces for the sides and good old velcro for the closures.