Flat Field to Remove Vignetting?

Hi,
Can someone tell me exactly how to use a flat field to remove
vignetting in my images, or point me to a tutorial on the subject? I'm
using a CCD video camera (StellCam) on a Meade LX-90 @ f/3. I am
experiencing a greyish round "haze" over most of the images, with the
edges/corners much darker. Stars still show up at the edges/corners,
but the sky background is much darker. It seems that this is
vignetting, and I'd love to know how to reduce or eliminate it.

Thanks in advance