The first and most important point that should be made is that exommunication, be it pronounced by the Church or not is something someone does to themselves. If someone does something worthy of the penalty, they are excommunicated or barred from the reception of communion. They remain a Catholic, just as any baptized Catholic does who has not made a formal declaration that they renounce the Faith.
The rites surrounding excommunication are usually reserved for someone who has made statements or preformed actions that clearly show their rejection of a moral or dogmatic teaching of the Church in a public manner, and who’s public actions would lead to the loss of Faith among others.
Example One: A Catholic goes to the Justice of the Peace to marry disregarding the laws of the Church, and treats marriage as a simple contract that can be broken. By doing so with no good reason, they are excommunicated even if no official statement or rite is preformed. They can be restored to communion by approaching the pastor of their local parish and having the marriage reconciled with God’s law and the Church if they plan to remain married. There are however some exceptions. In England the law used to be (and still may be) that a Civil Marriage must take place to be legal, and a Religious Marriage is optional (according to the State) Here the Church allows for a civil marriage to satisfy the State, and then the couple is married in the Church.
Example Two: A bishop ordains new bishops without the permission of the Pope (Papal Mandate) and when there is no grave need. If there was no grave need for a new bishop being ordained, with a situation making it impossible for the Pope’s permission or communication with the Vatican both the ordaining and ordained bishop are excommunicated, and they would not have the excommunication lifted until they straightened out the situation with the Pope, or someone the Pope delegates to judge the case.
As the subject of Martin Luther came up on another thread. Luther was given several chances to explain his new doctrines, including being invited to do so before the Council of Trent. He was promised safe conduct to and from the Council. He steadfastly refused. It seems from his own history he should never have become a priest, as he made a vow to enter monastic life when he was frightened during a thunder storm. Vows such as his made out of fear are not held to be valid, as they are made under diress. The Book Young Luther, by the renound Psychotherapist Erik Erikson is a good read about the instability of Martin Luther. His doctrines changed radically throughout his lifetime. He was given to physical and verbal fits where he would lash out at others, and would use very blue language, some of the excommunications that Luther and Lutherans wrote to Calvinists and Calvin and his followers against Luther and Lutherans would shock most who regard them as holy and “Godly” men. Even so Luther up to the moment of his death could have been received back into the Church by a simple confession to a priest or even a statement of contrition.