What is prescribed upon repentance in terms of supplications, prayers, or specific formulas and numbers for seeking forgiveness, and how does one remain steadfast in repentance thereafter?

One who repents from a sin is like one who has no sin. It is prescribed for the repentant person to praise Allah and thank Him for guiding and enabling them to repent, and to seek His forgiveness in any wording. The best form of seeking forgiveness is to say: "O Allah, You are my Lord. There is no god but You. You created me, and I am Your servant, and I am abiding by Your covenant and Your promise as much as I am able. I seek refuge in You from the evil of what I have done. I acknowledge Your blessings upon me, and I acknowledge my sin. So forgive me, for indeed, none forgives sins except You."

It is also prescribed for them to purify themselves and pray two rak'ahs, and to supplicate with duas such as: "O Allah, Turner of hearts, keep my heart steadfast on Your religion." Among what helps them remain steadfast after repentance is to distance themselves from bad environments and evil companions and replace them with good ones.