Shariah ruling about making up missed fasts
1) If a person eats or drinks something by mistake during the fast then case there is no making up or expiation on it, whether it is a fast of Ramadhan or not.
2) If a fasting person knowingly eats or drinks, then expiation and making both are compulsory.
3) And if a person fasting in the holy month of Ramadhan due to some reasonable circumstances such as in journey or because of sickness, breaks the fast, then only the making up will be liable on him/her but not the expiation.
The order of expiation is only for the fasts of Ramadhan. If someone invalidates the Supererogatory Fast due to some reason then in such case there is no expiation binding on him/her. Only making up the fast is sufficient.
Allah Most High says: (Fast) a certain number of days; and (for) him who is sick among you, or on a journey, (the same) number of other days; (Surah Baqarah: 184)
B: Journey here signifies that the duration of someone's journeys shouldn't be less than three days’ worth of distance. In this verse Allah Most High gives the concession to the sick and the travelers that if fasting leads to exacerbation of their sickness or poses threat to their lives or if one faces undue hardship during the journey then one may not fast in those days. Later on, except those days in which fasting in prohibited, one can make up the missed fasts. The prohibited days for fasting are: The days of the 2 Eid and 11th, 12th and 13th days of the month of Dhul Hijjah.
Regarding the making up of missed fasts of Ramadhan it is mentioned in Hidayah that the person who has to make up the fasts has the choice to fast continuously or as per convenience. However, it is Commendable (Mustahab) to make up the fasts of Ramadhan continuously so that the Obligation is executed at the earliest. If a person delayed so much in performing the qaza of them that the next Ramadhan arrived, then it is necessary that such a person should perform the Obligatory (Fardh) fasts of Ramadhan and should not try to make up any fast in this blessed month. After the month of Ramadhan has ended, that person should make up the missed fasts. It should be remembered that delay in making up the fasts does not impose any kind of penalty on the person. Simply making up the fasts is enough.