Brachot on Mitzvot

General laws of Brachot HaMitzvah

1. As a Brachat HaMitzvah it must be said Over LeAsiyatan, prior to the action of the mitzvah. Additionally this forbids any interruptions between the Bracha and the actual Mitzvah.Nonetheless, an action that's necessary to do the Mitzvah isn't considered an interruption between the Bracha and Mitzvah. For example, after makingt he bracha on Chanuka candles it's not an interruption to get candles or a match because these are nessecary elements for the Mitzvah.