Supplicating to Allah for one's parents

Q 2: A man asked the Messenger of Allah saying, "O Prophet of Allah! Can I be dutiful to my parents after their death?" The Prophet (peace be upon him) replied, "Yes. You can pray for them, ask Allah to forgive them, and carry out their pledges." Related by (Abu Dawud and Ibn Majah). How can a person pray for his parents as mentioned in the Hadith of the Prophet (peace be upon him)?

(Part No. 24; Page No. 220)  A: Prayer in the Hadith signifies supplicating Allah (Exalted be He) to forgive them, showing mercy to them, admitting them to Paradise, saving them from Hell, and so on. A similar meaning may be found in Allah's (Glorified and Exalted be He) saying, Take Sadaqah (alms) from their wealth in order to purify them and sanctify them with it, and invoke Allâh for them. i.e. and supplicate Allah (Exalted be He) for them Verily! Your invocations are a source of security for them May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.