How would one make up for the missed fasting days and the expiation for someone who did not fast their obligation for several years and then wished to fast them?

"Whoever delays making up the days of fasting due upon them until another Ramadan begins, without a valid excuse, must repent to Allah Almighty and promptly make up the days. In addition to making up the days, they must offer an expiation (fidyah) by feeding one poor person for each day they delayed its make-up. This is the fatwa issued by six of the Companions. However, if the delay in making up the fast was due to a valid excuse—such as ignorance of the prohibition of delaying—then they are only required to make up the days they missed."