Set up your camera as for a picture. Remove the ground glass and look at the inside of the bellows. You will see if there are bright places, usually on the lower side of the bellows. Cut some pieces of black card stock and stick them in the folds of the bellows between the bright spots and the film plane. Don't glue them in because the locations of the flare may be different for other setups. Obviously, the cards should not impinge on the field of view. A little playing around will give you the idea.