That's the way Jesus set it up on Easter Sunday evening.  He told his apostles that just as God the Father had sent him, so he was sending them.  He breathed on them the power of the Holy Spirit, giving them God's power to forgive sins, since no one can forgive sins but God alone.  He told them that whatever sins they forgive are forgiven and whatever sins they retain are retained.  (Jn 21:21-23; Mk 2:7).  Since the apostles were unable to read minds, the only way they would  know which to forgive and which to retain is if people told them their sins.  Jesus thus established the essential structure of the sacrament of confession.  Just as he uses priests to give us the Body and Blood at Mass so he uses them to give us His Mercy in Reconciliation.

The sacrament consists of three actions on the part of the penitent: contrition, which is sorrow for one's sins; confession, that is examining one's conscience and telling one's sins to the priest; and penance, namely a desire to make up for one's sins and amend one's life.  Together the three are called the "acts of the penitent."

One does so by asking for God's help and prayerfully examining one's actions in the light of Christ's teaching and example.  Some review their behavior by using the Ten Commandments as their guide, others by Christ's command to love God and others, others by their correspondence to the Beatitudes.  And others by comparing their behavior to the most important Christian virtues, and others by the seven "capital" sins (pride, envy, anger, sloth, greed, lust and gluttony).  It is not enough merely to focus on how one has broken God's "rules," but how one has damaged his relationship with God and others and has hurt himself inside.  It is also important to remember sins of omission.

A mortal sin involves an action whose object is grave matter that is committed with full knowledge and deliberate consent.  Grave matter is generally understood to be something that violates the Ten Commandments.  Full knowledge means that one is aware that God or the Church he founded considers the act sinful (even if one doesn't totally understand why it is sinful).  Deliberate consent means a consent sufficiently intentional to be a personal choice (CCC 1857-1859).

"When the priest celebrates the sacrament of Reconciliation, he is fulfilling the ministry of the Good Shepherd who seeks the lost sheep, of the Good Samaritan who binds up wounds, of the Father who awaits the prodigal son and welcomes him on his return, and of the just and impartial judge whose judgment is both just and merciful.  The priest is the sign and instrument of God's merciful love for the sinner."  The priest is called, practically, to lead "the penitent with patience toward healing and full maturity.  He must pray and do penance for his penitent, entrusting him to the Lord's mercy." (CCC 1465-66).

The Sacrament of Reconciliation is a spiritual resurrection bringing us to life after we were dead through mortal sin (Lk 15:24).  It reconciles us with God in loving friendship and restores us to his grace.  It is normally followed by peace, joy, serenity and spiritual consolation.  The sacrament also reconciles us with the Church, repairing or restoring the damage our sins have done to our communion with others.  The sinner also hopefully anticipates the judgment that will come at the end of his life, choosing the path of life over the path of death. (CCC 1468-1470).