As you hit the target, it takes an extra 1d6 Fire damage from the attack. At the start of each of its turns until the spell ends, the target takes 1d6 Fire damage and then makes a Constitution saving throw. On a failed save, the spell continues. On a successful save, the spell ends. Using a Higher-Level Spell Slot. All the damage increases by 1d6 for each spell slot level above 1.
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.