To calibrate a Weston meter, first zero correct it as described by Benjiboy. Then, on a bright cloudless day between 10 am and 2 pm, point it up at the clear blue sky in the opposite direction of the sun -- ie[the southern sky if you're in the southern hemisphere, north if you're in the north -- and take a reading. The needle should be at about 13.5, corrresponding to an exposure value of around 14 to 15.