If you missed prayers for a reason, there is no sin on you but you have to make it up. If you missed prayers with no excuse, you have committed a major sin and you have to repent to Allah. It is not valid for you to make it up according to the more correct of the two scholarly opinions.

It is recommended to make up missed prayers in congregation. In the battle of Al-Khandaq, Prophet Muhammad (peace and blessings of Allah be upon him) missed four prayers, and he made them up in congregation.

If you have prayed the current prayer, then you remember a missed prayer from the same day or a previous day, you should pray it, but do not repeat the current prayer. There is a difference of opinion among the jurists as to remembering a missed prayer during the current prayer. To be on the safe side, complete the current prayer, then make up the missed prayer, then repeat the current prayer.