Eurhm I've got a really lame question for the faq that isnt covered;
how and where do you use magic?
Magic can be acquired though research, though sovereigns all start with some spells. Researching elemental magic Books will unlock more spells.
Spells can be cast in two situations. Some spells are combat related and can only be cast in Tactical Battle. Other spells are world spells, and can be cast on the map. Spells can only be cast by the sovreign, possibly his/her spouse if that unit had any to start with, and by the resulting offspring.
Click on one of these units on the world map and on the bottom right, there should be an action option of [Cast Spells]. Clicking on that brings up the book panel from which you can select the spell you wish to cast and the area you wish to target.
Spells require mana, and if you do not have the required mana, that spell in the book will be grayed out. Some spells are a constant or indefinite, and as such, will continue to consume mana for every season after it's casting. These can be dispelled by clicking on the unit that cast the spell and on the bottom right click on [Enchantments]. From this list you can manage any long term spells that you may need to let go to free up mana.