How does a person who has not paid their zakat for previous years later fulfill their zakat debts?

If a person who was obligated to pay zakat in previous years did not do so, they must pay the zakat for those past years as long as they still possess the wealth. For example, someone who missed paying zakat for two years should first pay the zakat for the first year, and then give 2.5% of the remaining amount as zakat for the second year. (Mevsılî, el-İhtiyâr, 1/101; Ibn al-Humam, Fath al-Qadir, 2/165)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu