Instructions
Hold down the button you want to associate with a magic spell. If you are playing the PC version, this will be one of the numbered keys on your keyboard. For Xbox 360 players, hold down one of the buttons on your controller. After about two seconds, the Quick Map menu will appear.
Go through the list of spells and items that you are allowed to quick map to your keyboard or controller.
Click on the spell you want to associate with the key you held down earlier. Once you click a spell, it will be linked to the key or button. To change this, hold that button down again and select a different spell.