How can one make up for and expiate 14 missed Ramadans, knowing that I have not fasted them since puberty?

You must make up for the fasting you missed during that period, as it is a debt upon you, according to the Prophet, peace and blessings be upon him: "Allah's debt is more deserving of being fulfilled." The making up should be done according to one's ability. Expiation (kaffarah) is only required for those who broke their fast by sexual intercourse. Similarly, you must make up for the prayers you missed during that period, according to the majority of scholars, or suffice with striving in repentance and increasing in seeking forgiveness for those who deliberately abandoned them. However, if you were ignorant of the religious ruling, then you are not required to make up for any of that.