There are many ways to create a custom spell. I will now tell you the easyest one.
1.
Go to the objects > Abilities menu in the world edit.
2.
You will need a spell that you can base your spell on.
Press the "ceate new ability" button in the top of the window.
Choose the spell you want as base spell and name it.
If you want a spell there you want to trow a sheep at another hero you can base it on "Storm Bolt".
3.
Now you can modefy the spell chanching lots of things including the big hammer to another model ex: a sheep.
4.
Now you have to give the spell to a unit in your map.
Go to the Objects > Units Menu and choose the unit youb want to give the Ability.
In the top of the unit settings you will find Abilities (You will also find "Hero Abilities" if the unit is a hero). Double-Click on that field to change it. Click "add ability" or something like that and choose your ability from the list. To use the ability as "Hero Ability" go back to your spell in the Ability menu and Set the "is a Hero ability" Boolean to True.
zeeraw, theres a lovely thing called a typo. It should be fairly obvious that he meant 'DONT know how to use the object editor' (at least I hope thats what he meant )