Small blessed medals worn as a sign, not a charm.
Have the medal blessed by a priest. It takes ten seconds after Mass.
Wear it or carry it. Many people give one at a baptism, a confirmation, or a wedding.
The Miraculous Medal carries the prayer around its edge: O Mary, conceived without sin, pray for us who have recourse to thee.
A medal is a confession of faith you happen to be wearing. It does not act on its own. Its worth is that it keeps a truth in reach of your hand on a day when you would rather not remember it.
The design was described by Saint Catherine Laboure in Paris in 1830 and struck two years later. The nickname came from the public, not the Church.
Medals of saints and of the Cross long predate it. Christians have carried small marks of their allegiance since the catacombs, when doing so could cost you your life.
Wholly free. Any belief that the object itself protects you regardless of how you live is superstition and the Church rejects it.