How does one make up for missed days of fasting?

If breaking the fast was due to a legitimate Sharia-sanctioned excuse (such as menstruation, illness, or travel), then it is obligatory to make up the missed days, and the time for making up these days extends until the next Ramadan, whether consecutively or intermittently. It is not permissible to delay making up the fast beyond the next Ramadan, except with an excuse.

However, if breaking the fast was intentional and without an excuse: 1. If one did not intend to fast from the night before, then making up the fast is not valid for him; because fasting is a time-bound act of worship, and intentionally abandoning it invalidates its makeup. 2. If one intended to fast but then intentionally broke it during the day without an excuse, then he must make up that day. If the breaking of the fast was through sexual intercourse, then along with making up the fast, expiation (kaffarah) is also obligatory.

Whoever intentionally broke their fast must engage in sincere repentance and increase in righteous deeds.