Where do church musicians get their guidelines on what is proper music to use for mass?

There are several church documents that provide guidelines for Church Musicians. For a general guide on liturgy and music you can refer to Chapter VI of the Constitution on the Sacred Liturgy. More recently, the United States Conference of Catholic Bishops published Sing to the Lord, a document that provides musical guidelines for parishes and Church Musicians. For more information about these documents and other guidelines see the National Association of Pastoral Musicians website. There you can find all the necessary materials for planning and selecting music for worship.