Allah سبحانه و تعالى has made Salaah compulsory upon us. He states:
Indeed, prayer has been decreed upon the believers a decree of specific times
It is for this reason that any Fardh (compulsory) prayers that have been missed need to be offered (Qadha).
Following are some points pertaining to the Qadha Salaah, followed by the method of offering Qadha Salaah.
The Qadha of a Fardh Salaah is Fardh. Similarly, the Qadha of a Wajib Salaah is Wajib. There is no Qadha for any Sunan of Nawafil that have been missed completely.
Any Sunan of Nawafil that were commenced then become Wajib to complete. Therefore, the Qadha of terminated Sunan and Nawafil is Wajib.
There is no appointed time for Qadha Salaah. It can be read at any time (except for Makrooh times).
One should aim to complete all their Qadha Salaah as soon as possible. It is not required that a person performs the Qadha of Zohr Salaah within the current time of Zohr.
If a person has upto five Qadha Salaah remaining then it is necessary for him to complete his Qadha Salaah prior to performing the Salaah of the current day (and time). If he reads the current Salaah before his Qadha Salaah then it will be Wajib for him to repeat his current Salaah.
However, if the time for the current Salaah is coming to an end then he must first complete his current prayer and therafter perform the Qadha prayer(s).
If a person has upto five Qadha Salaah remaining then he should perform them in the order in which he missed them, i.e. Fajr first, followed by Zohr then Asr etc.
If someone has six or more Qadha Salaah remaining then there is no requirement for him to perform his Qadha Salaah prior to his current Salaah. Similarly, there is not requirement to complete the Qadha Salaah in the order that he missed them.
Therefore, it would be valid for him to start completing all his Fajr Salaah then all his Maghrib Salaah. Similarly, it would be valid if at one time he offered the Qadha for Zohr and at another time the Qadha for Eisha.
If a person missed his Fajr Salaah of the day and he is performing the Qadha of that Fajr on the same day, prior to midday, then he should also read the two Sunnah before the Fardh Salaah. However, if he performs the Qadha after midday then he should only do Qadha of the two Fardh of Fajr.
[Masail taken from Darsi Behesti Zewar]