How does one calculate and pay the zakat owed by a person who has not paid it for more than ten years?

It is forbidden to delay the payment of zakat when one is able to do so. For whoever does not pay it, it remains a debt upon them and does not lapse with the passage of time, by agreement of the four madhhabs. Therefore, they must pay zakat for all the past years in which they did not pay their zakat.