Make a new document (100 x 100). Make a new layer. Draw a circle with the
Elliptical Marquee tool. Save your selection (Select>Save Selection>Click OK)
Fill with medium gray. Feather by 4 pixels. Inverse selection and press
the "M" key. Now move the selection 2 pixels to the right and 2 pixels down,
with the arrow keys on the keyboard.
Now go to Edit>Fill>100% White, click OK.
Move selection up 4 pixels and to the left 4 pixels. Fill with
black (Edit> Fill>100% Black)