I wear a Brown Scapular. I chose it because I read that if the wearer goes to Purgatory than they will be delivered on the Saturday after they get there. I had to have a priest bless it with a special blessing for this promise to hold true and I pray the Rosary every day and that counts as my penance, or whatever you call it so that it works, however this is because I asked a priest permission for that to be. If you want it to work without asking a priest’s permission than you have to either abstain from eating meat on Wednesday and Saturday (if you don’t eat meat, I’d say that’s not a good penance for you,) or prayer the Little Office of the Virgin Mary every day, which consists of two long prayers each day–one for the morning and one for the evening. You can find them online. If you don’t mind asking a priest’s permission than you may ask to substitute any prayer or good deed you want.
Also, one must live a chaste life in order for it to work and also one must at least try to live in a state of Grace. Regardless, if it is blessed with the special blessing, it will ward away demons.