How to regenerate mana
Roleplay meditating or focusing for 30s. You can't make major movements, or interact with people beyond telling them to go away. At the end of this 30s, you can regenerate mana up to your regen value that you spent since the last time you regenerated mana. Regenerating mana ends any and all ongoing spell effects you have up, such as floating armour.
So what's my regen value?
Your regen value is calculated exactly like your hits. Add up your number of levels of Elemental Skill, Might, and Power. You have one point of mana regen for every four you have in any combination.