How can one atone for a period of non-adherence to the شرعي (Sharia-compliant) hijab, prayer, and fasting Ramadan after puberty?

May Allah bless you for your commitment to the Shar'i hijab and your observance of prayer. Know that there is no sin from which its perpetrator repents with sincere repentance except that Allah accepts his repentance, as supported by the noble verse: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful'" (Quran 39:53), and by the noble hadith: "Indeed, Allah extends His hand during the night to accept the repentance of the sinner of the day, and He extends His hand during the day to accept the repentance of the sinner of the night, until the sun rises from its west."

As for missed prayers, the view of the majority of scholars is that you must make up all the prayers you missed since the age of puberty. If you do not know their number, then make up what you believe, most likely, will clear your conscience.

Regarding fasting, you must make up the days of fasting you missed. If you do not know their number, then fast the number that you believe, most likely, corresponds to the missed fasts.

You are advised to increase in good deeds, for good deeds erase bad deeds, and whoever repents, Allah accepts his repentance.