If you have a Spell prepared that has the Ritual tag, you can cast that Spell as a Ritual.
This means it can be either casted as an using the spell's Casting time (usually within the turn) and using the Mana equal to the Spell level, or alternatively when cast as a Ritual Spell, it takes 10 minutes and uses no Mana.
While Casting in this way, the creature gain the status effect of Casting.