1- what do you meen by how do you mod?
2-what karouk said
3- go to object editor, click under abilities, click on make custom ability and it will give you a list of every ability in the game, click on one that is most like one you have thought of, for example lets make an ability where the caster will cast out a fire ball at everyone near him.
so for that example, select fan of knives, get rid of the ART effect which is what appears on the unit when he casts it, then click on the art missle and make it a fire ball, you can edit the missle speed, make it 1500 which is kinda fast, you can edit the damage, the area of effect, how much mana it costs and all that stuff.
if you didn't understand much of what i said your too noobish to be asking for abiltiy help, you should mess with it yourself first, gain a small understanding of it. good luck
