Stick it to the inside of the circle that you cut out earlier.
 You want it to stick out on either side of the circle about an inch and a half (2.5-3cm) or so. Start attaching the already taped scored piece of foamcore, as shown in the image, gently curving it around to fit on the circle piece.
 Cut right to where it joins the foamcore. Doing this allows you to eliminate any bumps in your taping that may otherwise be caused from taping the circle.
 Periodically, put it on the lens to make sure that it is holding its shape. You want to make sure that it will fit snugly on the lens and, yet, not be too snug.
Apply the tape that you are using.
Stick a piece of tape across the other side of the circle that you just taped.
Cut the piece of tape that is still jutting out along its center.
Continue doing this all the way around.
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