Hi, 

To complete a roll off in netball follow the steps below:
1) Drive out in one direction
2) Put your inside foot (if driving left your right foot and vice versa) out and turn your back to the defender 
3) You will push off from this inside foot, turn fully and drive out at an angle - this will also cut the defender off 

To practice this you can first begin to with a static defender, the attacker can simple go through the movement and how this feels. You can progress the drill further by allowing the attacker to run up to the defender and complete the roll - this will allow them to experience it on the move. Finally allow the defender to move to make it more realistic. 

Hope this helps.