“When making the sign of the cross, he holds the palm of the right hand turned toward himself, with all the fingers joined and held straight, and makes the sign of the cross by moving this hand from head to chest and from left shoulder to right.”
The 1984 Ceremonial of Bishops, n. 108, footnote 81, quoting the 1962 Missale Romanum (Ritus servandus in celebratione Missae, III, 5). It is in Chapter 4, General Norms.
The Ceremonial of Bishops also has that the bishop should “place his left hand on his breast, unless he is holding something” (n. 108).
From Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 44.
From the General Instruction of the Roman Missal, which applies to the Mass:
“275. A bow signifies reverence and honour shown to the persons themselves or to the signs that represent them. There are two kinds of bows: a bow of the head and a bow of the body.
· A bow of the head is made when the three Divine Persons are named together and at the names of Jesus, of the Blessed Virgin Mary, and of the Saint in whose honour Mass is being celebrated.”
So there is a bow of the head when the three Divine Persons are named together in the Sign of the Cross.