1. Create custom unit
2. Select a hero
3. Art - Model File option
4. Select a unit from the list (eg. archer)
5. If you place the hero, you should see an archer instead of say a paladin
6. Set the skills (hero or custom)
7. If its an ability, underneath hero settings there should be abilities (this is for effects you want to add such as roar)
For fun, I tend to make my own custom abilities. (with custom icons and everything.) You can do crazy things from casting holy light on yourself only! to stunning every enemy on a widespead area (99999 ft stun radius with war stomp)
Oh and if you want to see the arrows, you need to set the attack type to Missile from Normal. You can even make units fire possession animation ^^