Answer:
For qadaa fast to be valid, you must make intention (in your heart) of keeping qadaa fast at night before the time of Fajr salaat sets in. It is not necessary to keep all the qadaa fasts consecutively, although doing so is preferable so that one’s obligation is fulfilled as soon as possible. You may keep qadaa fast on any day besides the two Eid days and the three days after the Eid-ul-Adha, as keeping fast on these five days is impermissible.
One who has had the chance of making up for his missed fasts but did not do so until he is no longer able to do so must make a bequest for fidya (compensation) equivalent to the amount given in Sadaqatul Fitr to be given to the poor from his wealth in lieu of each day of fast pending upon him.
(Al-Ikhtiyaar: 1/194, Dar Quba)
(Raddul Muhtaar: 3/461, Darul Ma’rifa)
Mufti Faizal Riza