Here’s a video for a hook sweep from butterfly guard, using a single hook. If you can’t get the underhook with your arm, it gives you another option. It shows you how to go over his shoulder with your arm and sweep him with one butterfly hook. Another great option. This Start BJJ video is by World Champion Professor Rafael Lima.
I’ve included the video below for a bit of variety and to give you an alternative if your opponent protects his arm from being gripped. This video by Jeff Joslin of MMA Quickstart shows us how to do a butterfly sweep with a different finish to most. He shows us how to adjust grips and change the way your opponent goes over, to give you a surprise edge over experienced opponents.