You will need to be invested in the scapular. "Investiture
Conferral with the scapular indicates a special devotion and consecration to Mary. No one should undertake it who is not spiritual prepared to live in accordance with it.
The short form of investing or conferral consists of a priest or deacon taking a blessed scapular and while placing it over their head reciting with the person any Marian prayer (e.g. Hail Mary, Memorare, Salve Regina). The person is now invested. They don’t have any lists to join, though they are henceforth members of the Scapular Confraternity and share in its spiritual benefits (the prayers of the members). No special daily practices are obliged, though someone consecrated to Mary, of which the scapular is THE sign, should live chastity according to their state and recite the rosary daily.
There is also a long form in the Book of Blessings, chapter 46, which is very fitting for group investments. Every parish should have this ritual book.
For some years the Carmelite Order had permission from Rome to grant laity the Indult to enroll people. This permission is no longer given, but those who received the Indult in the past still have it. So, while laity may not bless a scapular, there are some lay people who can invest others, with a scapular previously blessed by a priest or deacon.
One final note, investing MUST be done with the cloth scapular. Those who wish to wear the medal can do so after investment. The scapular blessing attaches to each subsequent scapular. A new blessing is NOT required. This is not true of the medal, each of which must be blessed by a priest or deacon."
ewtn.com/expert/answers/brown_scapular.htm. So you will need to visit your priest to be invested. You might want to purchase a new scapular for this. If the old one is worn out and you are not sure if it had been blessed, you can dispose of it by burying or burning it.
