First you must choose which spell to cast. If you’re an adept, blackguard, cleric, druid, wizard, experienced paladin, or experienced ranger, you select from among spells prepared earlier in the day and not yet cast (see sub-section Preparing Wizard Spells of section Arcane Spells and sub-section Preparing Divine Spells of section Divine Spells).
If you’re an assassin, bard, or sorcerer, you can select any spell you know, provided you are capable of casting spells of that level or higher.
To cast a spell, you must be able to speak (if the spell has a verbal component), gesture (if it has a somatic component), and manipulate the material components or focus (if any). Additionally, you must concentrate to cast a spell.
If a spell has multiple versions, you choose which version to use when you cast it. You don’t have to prepare (or learn, in the case of an assassin, bard, or sorcerer) a specific version of the spell.
Once you’ve cast a prepared spell, you can’t cast it again until you prepare it again. (If you’ve prepared multiple copies of a single spell, you can cast each copy once.)
If you’re an assassin, bard, or sorcerer, casting a spell counts against your daily limit for spells of that spell level, but you can cast the same spell again if you haven’t reached your limit.