The white ring is made from white duct tape, and is completed in similar fashion to the center blue circle.
Trace the pattern piece labeled "WHITE CIRCLE" 8 or 9 times onto two strips of white duct tape, as shown in the second photo.
For placement of the white ring on the shield, I just eyeballed it. The goal is to have three rings of equal width (red, white, red). If it looks funny or crooked, you can always pull it up and re-do it.
A more precise method would be to use a ruler or measuring tape to place a few marks around the shield from the center point to aid in the placement of the white ring. The inside edge of the white ring should be placed about 7 1/8" (181 mm) from the center point of the shield. (Thanks to Iron-Man227 for this tip!)