Sometimes, it's best to start with a video to explain what a new, foreign concept looks like.  Here, we have a pretty decent explanation of rucking from Austin Rugby Club both from the attacking side and the defending side.  

What's most critical is the way the defensive side turns over the ball and takes advantage of the ball carrier falling improperly.  Remember, while you have to give the tackled player a chance to "play the ball", you do not have to let her roll over and hand the ball to her own scrumhalf.