It requires a lot of micro.
When you have your banelings loaded into your ovies, you hold on to the 'shift' key while you press 'd' around the areas you want to drop. However, don't select all your ovies in one group, and do that, because then you 1 ovie will drop 1 baneling at a time. Try to separate your ovies into 3 groups, and do that for each group of ovies.
However, your timing has to be really good. Too early, your overlords will get sniped. Too late, and your army will die before your banelings can assist.
If I was doing baneling drops, I would probably hotkey each group of ovies, do the 'shift, d' ordeal, then attack-move my army.
I don't normally do it though because I'm not that good at it though. Havn't practiced it enough.