If you create for example from Chain Lightning a custom spell and name anything you want and change stuff that it deals 20000 damage and jumps up to 12 people.
If you want to order some unit to cast this spell it's just done by the original order action, example for the Chain lightning i said:
Unit - Order Unit(you want it to do) to Orc Far Seer - Chain Lightning Target(target unit to cast it)
Meaning the order is same as the base ability you made the custom ability from