How should I treat a torn ligament?

Ligament dependent. Not all ligaments or degrees of ligament injury are treated the same. Some ligament injuries require simple supportive care (rest, ice, compression, elevation- rice). Others may benefit from a course of bracing. Still others may mandate surgical repair or reconstruction. Pending initial evaluation by a qualified healthcare provider, rice combined with immobilization is a safe plan of action.
With expert guidance. Appropriate treatment of a torn ligament depends a variety of factors including which ligament is injured, how badly it is injured, what other structures are injured, what the functional demands of the patient are, how much instability the injury causes, how old the patient is, what other medical problems the patient has, etc., and is best determined in consultation with qualified physician.