Here's the full explanation of the "speed stop" drill inside my OG Hoops Academy. To be an effective guard, you must be able to stop and change directions.
I know this is only one 20 second video, but I would spend a considerable amount of time working on each of these speed stops.
TIP: Start with 1 dribble full speed and then move onto 2 and 3 dribbles when it gets easy. Be sure to go full speed - you're only cheating yourself if you're not!