Break down rucking in terms of key factors and focus on them:
where to bind
how many players over the ball?
You can think of lots of exercises to practise these skills. Do not try to achieve everything in one exercise. On decision making, players have to ask themselves:
do I go over?
do I pick up and drive?
do I pick up and pop to a player who drives?
do I pick up and pass wide?
Again, think of exercises where players need to make these decisions, game based learning!