1. You see if you cast the spell by rolling against the appropriate Rune (for Rune magic), against your POWx5 (for spirit magic) or appropriate sorcery spell (for sorcery).
2. If the spell needs to overcome the target’s POW you roll on the Resistance table against that.
That parallels the two rolls you generally have in combat – first to see if you successfully used you weapons, and second to see if you opponent parried or dodged. The only difference is that in magic that caster rolls twice, which in combat each combatant rolls once.