Its kind of hard but it gets easy.
First make a blank spell (one that has no effects)
Then make a trigger
Events
Unit - A unit Begins casting an ability
Actions
Special Effect - Create a special effect at ((Position of (Casting unit)) offset by (0.00, 0.00)) using (effect you want)
The numbers there (0.00, 0.00), tell were it is going to happen. The first is Vertical Axis; the second is Horizontal Axis.
It helps if you know how to use a coordinate plain.
After that u can create another effect at the same time, or wait...
If that doesn't help, i could probably make it simpler or somthing.
I knew i forgot something! you need a condition! lol sorry... here it is...
Conditions
(Ability being cast) Equal to (Name of yur blank spell)