Ideally, debeaking machine (electric) should be used in debeaking the birds. However, this depends on the number of the birds. For instance, birds less than 200 pieces in number can be debeaked using a very hot knife. The use of scissors in really not advisable as the beaks can grow back very quickly.
See also: Debeaking of Birds, the Need for Debeaking and the Ideal time for Debeaking the Birds