You can simply start wearing it, it is no longer necessary to be formally enrolled, but you can be.
carmelitanacollection.com/catechesis.html
I wear the brown scapular as part of the fivefold scapular, and previously wore the brown scapular by itself, or the scapular ring. The wool and the moving around of the scapular, which gets unbalanced, is inconvienient, but I find that inconvenience is also a blessing as it is a reminder.
The spiritual benefits are from the wearer’s devotion, not the object. The Church grants indulgences, that is, remission of the temporal punishment due sin, for sins already forgiven. An indulgence may be obtained from using a crucifix, cross, rosary, scapular, or medal.
In 1967 Indulgences were revised (the latest revision is 1999), rescending all previously given, and requiring orders to re-establish them. The Carmelites did. So if you read about the indulgences for a particular scapular anywhere, keep in mind that the information given is only historical, if before 1967.
**Also, on specific days, wearers of a scapular and members of certain confraternities that wear them, are eligible to obtain a plenary indulgence, under the usual conditions of: intention, in the state of grace (at least by the time of completion), sacramental confession (within 20 days), communion, vocal prayers for the intentions of The Pope, and detachment from all sin (regret for even venial sins).
These are the days for the brown scapular indulgence:
The day of investiture. Feast of Saint Simon Stock, May 16
Feast of Our Lady of Mount Carmel, Jul 16
Feast of Elijah, the Prophet, Jul 20
Feast of Saint Therese of the Child Jesus (Lisieux), Oct1
Feast of Saint Teresa of Avila, Oct 15
Feast of The Saints of Carmel, Nov 14
Feast of Saint John of the Cross, Dec 14 **
The fasts are part of the Sabbatine Privilege which is not an indulgence granted by the Church. Some believe that this originated from Pope John XXII in the year 1322 in a vision of Our Lady of Mount Carmel, but it is without historical foundation. In 1613 the Holy See instructed the Carmelites to stop mentioning the “Sabbatine Privilege”, but that people may believe that The Blessed Virgin Mary will help them especially on Saturday, which is her day. Read about it here, exerpted from The Scapular of Our Lady of Mount Carmel: Catechesis and Ritual:
carmelitanacollection.com/catechesis.html
For Church indulgence, the scapular or the scapular medal can be used.