To use Magic, a character must have two things: the Arcana skill, and an Aspect reflecting their magical powers.
At its simplest, this can simply be a character class (eg cleric or wizard).
To cast a spell, use your Arcana skill to Overcome, Attack, Create an Advantage, or Defend (against Magical Attacks only)
A magic-using aspect is also necessary to do non-combat magical things like activating a teleportation circle or setting up a whispercall; if time or degrees of success are of the essence, this would be an Overcome action, otherwise simply having the aspect will suffice.
Invisible Shield – you can use your Arcana skill to defend against physical attacks.
Mental Fortress – you can use your Arcana skill to defend against mental attacks.