How should someone who forgets the first sitting in obligatory prayers complete their prayer?

The first sitting (qa'da) is one of the necessary (wajib) acts of prayer. If a wajib is forgotten, prostration of forgetfulness (sajda sahw) must be performed in the final sitting. Deliberately omitting the first sitting is severely disliked (tahriman makruh), and thus the prayer must be repeated. (Ibn Nujaym, al-Bahr, 1/310; Ibn Abidin, Radd al-Muhtar, 2/88)

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