Mediator Dei on the Vatican web site–read more there any time.
Today, the founding of what today is known as the CDWDS, the Congregation for Divine Worship and the Discipline of the Sacraments:
57. The Church has further used her right of control over liturgical observance to protect the purity of divine worship against abuse from dangerous and imprudent innovations introduced by private individuals and particular churches. Thus it came about – during the 16th century, when usages and customs of this sort had become increasingly prevalent and exaggerated, and when private initiative in matters liturgical threatened to compromise the integrity of faith and devotion, to the great advantage of heretics and further spread of their errors – that in the year 1588, Our predecessor Sixtus V of immortal memory established the Sacred Congregation of Rites, charged with the defense of the legitimate rites of the Church and with the prohibition of any spurious innovation.[Constitution Immensa, January 22, 1588] This body fulfills even today the official function of supervision and legislation with regard to all matters touching the sacred liturgy.[Code of Canon Law, can. 253]
The Pope may begin a new practice, recognize an innovation, or modify the liturgy. I presume this was understood to take place not by random practice, but by a reasoned decision:
58. It follows from this that the Sovereign Pontiff alone enjoys the right to recognize and establish any practice touching the worship of God, to introduce and approve new rites, as also to modify those he judges to require modification.[Cf. Code of Canon Law, can. 1257]
Lay people and others may or may not dispute this, given the furor in some quarters over the addition to St Joseph to the Roman Canon in the days of Pius XII’s successor.
Bishops, for their part, have the right and duty carefully to watch over the exact observance of the prescriptions of the sacred canons respecting divine worship.[51. Cf. Code of Canon Law, can. 1261]
Bishops police the liturgy, in other words.
Private individuals, therefore, even though they be clerics, may not be left to decide for themselves in these holy and venerable matters, involving as they do the religious life of Christian society along with the exercise of the priesthood of Jesus Christ and worship of God; concerned as they are with the honor due to the Blessed Trinity, the Word Incarnate and His august mother and the other saints, and with the salvation of souls as well. For the same reason no private person has any authority to regulate external practices of this kind, which are intimately bound up with Church discipline and with the order, unity and concord of the Mystical Body and frequently even with the integrity of Catholic faith itself.
Private individuals cannot decide matters dealing with liturgy in the context given here: nobody I know, at least no serious liturgist, has a problem with this.