Unfasten the screws holding the web straps on, slide off washers and set aside. Slide the scissors underneath the loop of webbing that connects the strip of webbing to the chair frame. Cut through the webbing. Repeat at the opposite end of that strip. Remove the strip of webbing and set aside. Repeat for all additional webbing strips that need to be replaced.
Unroll the new polypropylene material onto a flat work surface. Lay one of the strips of webbing on top of the material. Trace around the old webbing strip with a pen and then discard the old strip. Add 2 inches to the traced strip and cut it out of the new material . Set aside.
Lay the new strap flat on the work surface. Form a triangle at the end of the strap by folding the corners 2 inches up. Secure the folded edges in place with a small piece of duct tape. Drill a hole into the center of the triangle. Hold the new strip up to the chair frame and stretch it across to make sure it fits before repeating the triangle and adding a hole to the other end of the strip. Repeat the entire process on all of the new strips.
Place the strip on the chair frame and wrap the end of the strip around the frame. Position the strip with the folded triangle on the underside of the chair frame. Line the hole in the folded triangle up with the hole on the chair frame. Secure the strip in place with a screw and washer. Stretch the strip across the chair frame. Wrap it around the frame and secure as directed above.
Open the chair and fold it shut several times to ensure that the strip is not too tight or loose. Make any necessary adjustments. Work from the back to the front of the chair, adding additional strips and continuing to check for proper fit after every strip is added. Weave strips between other existing strips as you go to create a weaved pattern.