assalamualaykum w w
الجواب وبالله التوفيق
In order to perform qadha(i.e. to make up for the missed salats) for a period of one’s past life, at the time of making the intention of the missed salah, one should state that I intend to pray the first ever (or the last ever) fajr salah which I missed and then continue praying it in this sequence. Similarly the intention should be made for other qadhasalats such as zuhr, asr etc. in the same manner. One should perform the calculation of the total number of missed salats based on the number of total years (or months or days) and perform qadha for that number. Otherwise, one should continue on reading qadha until one’s heart becomes satisfied regarding completely making up for their missed salats.
واللہ اعلم بالصواب