Start with a new 640x480 file.
Make a new layer and fill a circular selection with white.
Tip – when making the selection hold down the shift key.
Then hold down the Alt key to center out.

Press the lock transparent pixels icon.
Deselect (Ctrl D)

Hit D for the default colors
(Black in the foreground.)
Make a new brush;
Base it on Soft round 100 pixels.

Set Other dynamics/Opacity Jitter to Fade
Turn on Airbrush

You can hit the new brush icon and name your brush:

Set the brush to a very low opacity:

Slowly build up with black to get something like this below,
If it starts getting “blobby” do a Surface Blur.
Tip – hit Ctrl F whenever you need to apply the blur again.
Use White if you need to re-cover any area.
Tip – Hit X for white to come into the foreground.
