We make reparation to Mary for specific insults and blasphemies committed against her, such as blasphemies against her virginity and blasphemous images of her.
“How do we do it” - by doing the First Saturday devotion, which has specific elements (confession, Holy Communion, a rosary, meditation on the mysteries). Note that all these things are good practices that we should be doing anyway, so the devotion helps us grow in our faith and become closer to Jesus.
“Is the idea based on some requirement that each person must make reparation to Mary” - It’s an optional devotion. It’s along the same lines as the Pope and bishops calling for us all to fast and pray for the sins committed by clergy in the abuse scandals. We make reparation for the sins of those who insult and blaspheme. If we personally don’t commit such sins ourselves, we can offer our reparation for the sins of others who do.
“Reparation to Mary is not finished then until each person does it?” This doesn’t make sense. It’s an OPTIONAL DEVOTION. I think it would be a nice thing if every person did it, because I think being devoted to Mary is a beautiful thing and it has certainly helped me personally grow in my faith in addition to making Mary and Jesus happy. But the Church does not require anyone to do it, and it is unlikely every person in the world (which would include a lot of non-Catholics and non-believers) is going to do it. Even if everybody did do it, I don’t get what you mean by “finished”. Probably when we’re dead, our earthly devotions finish and we go on to new devotions in Heaven. Apart from that, there is no set time for “finishing” a private devotion of reparation, as presumably new sins are being committed every minute.
“What about people who never heard of making reparation to Mary?” Well, obviously they aren’t going to be doing it then, are they? Those of us who have heard about it, do it, and try to spread the word to others. As I have stated multiple times, it’s Not A Requirement of the Church, It’s an Optional Devotion, so people can choose to do it or not.
“I am troubled about the message of Fatima…” OK, so that’s your issue. You don’t like the devotion, don’t do the devotion.
Literally every optional devotion on here, including Rosary, Divine Mercy, Fatima devotion, etc. has somebody popping up saying “I am troubled about this devotion because (whatever)”.
I don’t see what the big deal is when no one is compelling you to do it or said you have to do it. If other people want to do it, then it’s not your issue or concern.
I frankly don’t much care if you’re troubled by it.
