I think I learned the way I do it from Nancy Zieman. I use the light interfacing method. I use fusible interfacing.
Sew around your circle, making sure that the side with the fusible is towards the right side of the fabric circle. cut the slit in the interfacing, turn and use a wooden or plastic seam presser and run around the edge, BECAUSE YOU CANNOT PRESS THE CIRCLE DUE TO THE FUSIBLE. When you turn the circle the fusible will be on the outside.
Then center the circle on your "plate", NOW YOU CAN PRESS. Most of the time, I leave the full interfacing circle on the back....some people trim to a 1/4 inch, leaving a small amount all around the edge to fuse down. I have found the light weight fusible interfacing doesn't add that much to the weight.