So, I've seen this mentioned in a few threads now, and people seem to be having a hard time with this and there seems to be a lot of confusion. So, just to clarify, this is how it's done:

1. Add the item/spell to your favorites.
2. Open the favorites menu and equip the item to your left hand.
3. While the item is equipped assign it to a hotkey.
4. Close favorites menu.

Now, whenever you press the hotkey the item/spell will be equipped to your left hand.