It is not as accurate as hydrometry as mentioned above....but "good enough" to follow the progress of a fermentation.
I have prepared a document describing the process and procedure:
The refractometer approach's biggest problem is that it cannot be used to reliably to indicate dryness (end of fermentation). However, even a hydrometer comes up short here. As an amateur winemaker I rely upon Clinitest tablets to estimate the residual sugar level indicative of dryness. As always, there are many ways to skin this cat.