How should I make up the fast that I missed during my (menstruation) periods over the years?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
You should make an assessment according to your best judgement and calculate the number of days of fasts that you think that you have missed. Then you should organize a system to cover up these days. Organize a system that is easy for you to follow.
It is sufficient to make the intention that you are keeping qada fast the first in your obligation. Intention for qada fast must be made the night before observing the fast otherwise the fast will not be valid and it will be regarded as nafl fast. (Bahishti Zewar Vol. 1 pg 332)
Fasts that have been missed should be made up as soon as possible. It is a sin to delay your missed fasts.(Ibid)
And Allah Ta’āla Knows Best;
Checked and approved,
Mufti Ebrahim Desai.