First, this is an illicit practice. According to Redemptionis Sacramentum:
[76.] Furthermore, according to a most ancient tradition of the Roman Church,*** it is not permissible to unite the Sacrament of Penance to the Mass in such a way that they become a single liturgical celebration. ***This does not exclude, however, that Priests other than those celebrating or concelebrating the Mass might hear the confessions of the faithful who so desire, even in the same place where Mass is being celebrated, in order to meet the needs of those faithful.158 This should nevertheless be done in an appropriate manner.
I’m hoping that what you described is not a joining of the Sacrament of Penance with the Mass. Now, communal reconciliation services are allowed and encouraged, however, they must adhere to these guidelines:
In accord with the law and practice of the Church,*** the faithful must orally confess their sins (auricular confession)9, except in cases of true physical or moral impossibility (e.g., extreme illness or physical condition inhibiting speech, speech impediment, etc.) This disposition would exclude communal celebrations of the sacrament in which penitents are invited to present a written list of sins to the priest confessor. It should be noted that such innovations also risk compromising the inviolable seal of sacramental confession.***
- In giving consideration to the authenitic discipline of the Church concerning “general absolution”, the recent interdicasterial meeting of the Roman Curia with a representation of Bishops of the Episcopal Conference of Australia noted that:
. . . communal celebrations have not infrequently occasioned an illegitimate use of general absolution. This illegitimate use, like other abuses in the administration of the Sacrament of Penance, is to be eliminated.
I realize that I’m probably giving you too much information, but, I believe that information and knowledge about the sacrament of penance are important, especially when you’re a newbie.
As far as the procedure is concerned, if this Communal Reconciliation Service is done according to what the Church requires, make your confession as you would do under normal circumstances. Depending on the set-up, you’ll either be face-to-face or behind the screen. Start off with “Bless me, Father, for I have sinned. This is my first confession.” The priest will take it from there and guide you along. Then, after you’ve confessed your sins, he will assign a penance for you to do. Then, he will, more than likely, ask you to pray the act of contrition. You can probably bring a small copy of the prayer with you. After you make an Act of Contrition, he will give absolution and then dismiss you.
I hope this helps.