Using the picture or diagram as reference, route the new webbing through the rings and buckles.

Fasten the new webbing to the harness by sewing it. In the image, note that the webbing has been routed through the buckles before sewing.

Since this is thick nylon webbing, it will be difficult to pin it down before sewing.

The existing harness is not reflective, though the new section is, which should enhance safety.