Step 1. Scan (DUH!!!). Scanned at 200dpi

Step 2. Crop with "rectangle" cursor. To make it easier, I blow up the scan 4 or 5 times to crop perfectly. I also crop so that the final size is an even numbered perfect square. In the instant case, the scan is now 310 x 310 pixels.

Step 3. Turn on the grid. Adjust the grid lines to interect at the center. In the instant case, horizontal and vertical lines were set to 155

Step 4. Crop with the "circle" cursor. Key point: turn on "snap position to grid". Now when you put the cursor in the center and draw out, the the circle snaps to the 4 sides resulting in a perfect circle with a diameter of 320 pixes. I then paste into a 340 x 340 size box. Of course on my site I would use a white box (for ivories I use a black box) QED.