Sailrite, Funky Little Chair, and Fabric Farms Interiors have extremely helpful videos on topics like how to use the silk film and how to use the Sailrite hot knife. Funky Little Chair's video was especially help and I used their instructions to make the box cushions.
Funky Little Chair video on making a box cushion - Box Cushion video by Funky Little Chair
Fabric Farms Interiors discussing how to cut welting fabric on the bias, but not have to much waste, by using a "60 inch bias" method where the piece is cut at only a slight diagonal. You use less fabric but still get the advantage of the give with a bias cut - Bias cutting by Fabric Farms Interiors