Two small pieces of cloth worn over the shoulders under clothing.
It must be enrolled once by a priest. After that a worn one can be replaced without repeating the enrollment.
Wear it over the shoulders, one panel front and one back, under your clothes.
Those who wear it are asked to live chastely according to their state in life and to pray daily, commonly the Little Office or a decade of the Rosary.
It is a habit, in both senses: a garment of Our Lady's order, and a daily reminder worn against the skin. Sacramentals work by disposing the person, not by acting on their own, so a scapular on a man determined to stay in sin does nothing at all.
The scapular began as the working apron of monks, the piece of cloth that hung front and back over the habit. To give a layman a scapular was to attach him to the prayer of the community.
The Carmelite form spread widely after the fourteenth century and became one of the most common devotional objects in the Catholic world.
The promises attached to it in popular tradition come from private revelation. The Church permits the devotion and has never made belief in any particular promise obligatory.
Free. It carries no guarantee, and treating it as an insurance policy is superstition, which the Catechism names plainly. Worn as what it is, it is a good and old devotion. A blessed scapular medal may be worn instead.