The act of preparing spells is quite easy. First, make sure you aren’t in combat. Then check a character’s spellbook, either by pressing “K” to pull up one character’s spells or by pressing the Tab button and selecting the “Spellbook” page to view everyone’s magic. There, spells will be divided into rows. If you’re trying to prepare spells, the only two you need to focus on are the ones with the book icon on the left (which is highlighted) and the row underneath it. The book icon row lists all of the spells that a character currently has prepared. To replace one of these and prepare another, simply click on a currently prepared spell (which should free up a slot). Finally, click one of the spells underneath the prepared spells row to fill the slot. And that’s it. You now have a new list of buffs and weaponized fireballs at your disposal.
Of course, that small tutorial only handles how to prepare spells, but now for the biggest question: How do you know what spells you should have prepared? That’s mostly up to you. Go for whatever you think will help in a situation or will fit the theme you’re trying to roleplay. Just know that there are a few additional rules, restrictions, and requirements to be aware of when you’re expanding your caster’s mystical arsenal.
For starters, certain classes don’t need to prepare spells whatsoever. Sorcerers and Warlocks, for instance, can use any magic they’ve memorized. As a tradeoff, they can’t learn as many spells as Wizards and are thus less of a Swiss Army spellcaster.
Furthermore, certain spells are always prepared no matter what regardless of class. These include Cantrips and spells linked to specific races and subclasses. For instance, as a Trickery Domain Cleric, Shadowheart comes equipped with Disguise Self and Charm Person. These abilities still require spell slots and long rests as normal, but they are otherwise freely available.
The final spell type you don’t need to prepare are Ritual spells. If you cast one of these outside of combat, it won’t require preparation beforehand, and it won’t eat up a spell slot, either. To compensate for this seemingly overpowered benefit, you can only cast a ritual spell once per long rest. If you want to use a Ritual multiple times per in-game day, you need to prepare it and use the requisite spell slots. Not an easy decision.