You must confess all mortal sins since your last good confession, and that would include receiving Holy Communion unworthily. That’s it, unless you have also made a bad confession, which means concealing sins on purpose. If you have done that, you need to confess all your sins again, back to your last good confession, including the fact that you made a bad confession.
The priest will have some words of advice, a penance and absolution for you, as in any other confession. There’s nothing special to be done as there might be if you had done something that resulted in excommunication.
Pretty simple. Not scary enough to keep a person from doing it. If you don’t get it done before Christmas, please do not receive Holy Communion, as it just piles up mortal sins for you and offends God. Not what you want to do on Jesus’ birthday.
Betsy