Do the missed days from fasting in previous years have to be made up by fasting only, or by fasting along with expiation? And in the event that expiation is obligatory, what is its amount?

It is not permissible for a Muslim to delay making up for Ramadan without a valid excuse until the next Ramadan begins; whoever does so commits a sin. They must make up the days they missed. The preponderant opinion is that no expiation (kaffarah) is required along with the make-up, but if one gives it as a precaution, there is no harm. Its amount is feeding one needy person for each day. They must also repent and resolve not to repeat this action.