BG3 spellcasting becomes much easier to plan once you separate four ideas: what spells a class can access, which spells are prepared or known, how spell slots are spent, and whether a spell requires concentration.
Spell slots are a resource
A powerful spell is not automatically the right choice if the encounter can be solved with a cheaper option. Save high-value slots for moments that change the fight.
Prepared versus known spells
Different classes manage spell access differently. Build planning should account for whether you can change prepared spells easily or are working with a smaller known-spell list.
Concentration
Only one concentration spell can normally be maintained at a time by a character. Avoid filling the whole plan with concentration effects that compete for the same slot.
Party role
Decide whether the caster is mainly damage, control, support or utility. That role should determine which spells stay available most often.
Positioning matters
Spell selection is only half the build. Range, line of sight and keeping concentration safe can matter more than one extra damage spell.