From the “Youcat – Youth Catechism of the Catholic Church”, translated by Michael J. Miller, published by Ignatius Press, 2011:
“272 What are sacramentals?
Sacramentals are sacred signs or sacred actions in which a blessing is conferred.
Examples of sacramentals are holy water, the consecration of a bell or an organ, the blessing of a house or an automobile …”.
Take the example of a blessing of an automobile. From the Book of Blessings, USA edition: “856 Whenever just one vehicle is to be blessed, the shorter rite provided is nos. 873-877 may be used.” In the Latin edition of the book, De Benedictionibus, this sentence is in n. 656. There is no Sign of the Cross in the ceremony, whether it is lead by a priest or a lay person. The rubric for the prayer of blessing is “875. A minister who is a priest or deacon says the prayer of blessing with hands outstretched; a lay minister says the prayer with hands joined.” Another rubric, at the end is: “877 As circumstances suggest, the minister may sprinkle those present and the vehicle with holy water.”
Some quotes from the General Introduction to the Book of Blessings:
“23. In the adaptation of celebrations a careful distinction must be made between matters of less importance and those principal elements of the celebrations that are here provided, namely, the proclamation of the word of God and the Church’s prayer of blessing. These may never be omitted even when the shorter form of a rite is used.”
“27. The outward signs of blessing, and particularly the sign of the cross, are in themselves forms of preaching the Gospel and of expressing faith. But to ensure active participation in the celebration and to guard against any danger of superstition, it is ordinarily not permissible to impart the blessing of any article or place merely through a sign of blessing and without either the word of God or any sort of prayer being spoken.”