Before you can use salt cod, it must be reconstituted. Rinse the cod well and then immerse it in a container of cold water (cut into chunks, if necessary, to fit). Refrigerate, changing the soaking water 2 or 3 times, until softened (12 to 24 hours, depending on how stiff it was when purchased). Drain the cod and place it in a large pan; pour in cold water to cover. Bring to a boil; reduce heat, cover, and simmer until fish is opaque and plumped and falls into heavy flakes when prodded (about 20 minutes). Drain and pat dry. Remove bones and skin, if necessary. Use in recipes or if made ahead, cover and refrigerate for up to 2 days.