When beginning a drill on the manual of arms, run through the manual of arms. Dismiss those who don't make mistakes. Don't penalize your men who attend all the events and know the drill, by making them drill with those who don't. Practice doesn't make perfect. Mediocre practice just reinforces bad habits.
Good practice makes for good drill.
If someone is doing it wrong, tell them. They can't do it right unless you do. The length of time you practice doesn't necessarily make your troops drill better. If you really want to drill them for two hours, do it in two one hour segments. Remember, even Stonewall Jackson marched his men only fifty minutes in an hour.