You have achieved such mastery over certain spells that you can cast them at will. Choose a level 1 and a level 2 spell in your spellbook that have a casting time of an action. You always have those spells prepared, and you can cast them at their lowest level without expending a spell slot. To cast either spell at a higher level, you must expend a spell slot. Whenever you finish a Long Rest, you can study your spellbook and replace one of those spells with an eligible spell of the same level from the book.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.