For those who have missed obligatory fasts from previous years, it is preferable to make up for those missed days first before observing the six Nafil fasts. However, if someone has only missed fasts during the recent Ramadan, they can start with the six Nafil fasts and later make up the missed obligatory fasts. Alternatively, one can choose to make up missed fasts during Shawwal itself, but these fasts will not be considered Nafil; rather, they will be the obligatory fasts owed to Allah. After completing these obligatory fasts, if they wish, they can observe the six Nafil fasts of Shawwal.