After very carefully cleaning the piece and drying it for several days I cut away the linen and backing from around the button (the fabric that held the embroidery in the hoop). You can see the sheer fabric in this photo that was left. Then the instructions said to sand the acid-free cardboard disk to bevel the edge. I next applied a cotton pad to the disk (again carefully trimmed) with supplied archival glue.
Here the piece is pinned to a tan linen backing, centered, and I started stitching a gathering (running) stitch around the button with the supplied Nymo thread. The running stitch was positioned about 10cm from the ladder stitch edge to allow for turning over the cardboard.
Here's the finished project!!! I'm so pleased with the results. I learned a great deal from Jenny's wonderful classroom instruction and fantastic written instructions.