How do I know when to cross myself during Mass (Novus Ordo)?

The short answer to your question on when the participant of the Holy Mass and you cite Novus Ordo make the sign of the cross is there are two types or classification. There are certain part on the Holy Mass that it is "required" and there are others that are called "optional".

Your question falls on the so called "optional". That's why the Mass participants have freedom to exercise their God-given freewill according to one's disposition.

For someone who understand the meaning of every part of the Holy Mass and when to acknowledge the meaning or significance of that part, making the sign of the cross is an outward pious gestures of accepting that part of the Holy Mass.

Now, for some Catholic members who are not well-informed on the Value and Sanctity of the Holy Mass and what graces a faithful can derived out of participating in the Holy Sacrifice of the Mass, unfortunately we cannot expect all participants to be on the same level of dispositions.

In this view, whether the participants make or not make the sign of the cross in the part where it is consider optional it will not in any way lessen his participation.But a well-disposed participants certainly will receive more graces than the latter.

Liturgical

Roman Catholicism draws a distinction between liturgical and non-liturgical use of the sign of the cross.

The sign of the cross is required at certain points of the Mass:

1.the laity sign themselves during the introductory greeting of the service,

2.before the Gospel reading (small signs on forehead, lips, and heart),

3.and at the final blessing;

optionally,

1.other times during the Mass when the laity often cross themselves are during a blessing with holy water,

2. when concluding the penitential rite,

3.immediately after receiving Communion,

4.and when concluding private prayer after Communion.

In the ordinary form of the Roman Rite the priest signs bread and wine once before the consecration. In the Tridentine Mass the priest signs the bread and wine 25 times during the Canon of the Mass, ten times before and fifteen times after they have been consecrated.

The priest also uses the sign of the cross when blessing a deacon before the deacon reads the Gospel, when sending an Extraordinary Minister of Holy Communion to take the Eucharist to the sick (after Communion, but before the end of the Mass), and when blessing the congregation at the conclusion of the Mass. Ordained bishops, priests and deacons have more empowerment to bless objects and other people. While lay people may preside at certain blessings, the more a blessing is concerned with ecclesial or sacramental matters, the more it is reserved to clergy.[16] Extraordinary ministers of Holy Communion do not ordinarily have a commission to bless in the name of the Church, as priests and deacons do. At this point in the liturgy, their specific function is to assist the clergy in the distribution of holy Communion.[17] Extraordinary Ministers of Communion, blessing those who do not wish to or cannot receive communion can speak or raise the hand but not make the sign of the cross over the person.[18]

Non-liturgical A priest or deacon blesses an object or person with a single sign of the cross, but a bishop blesses with a triple sign of the cross. In the Catholic organization the Legion of Mary, members doing door-to-door parish surveys bless the homes of those not home by tracing the sign of the cross on the door

https://en.wikipedia.org/wiki/Sign_of_the_cross

In closing, God the Father thru Jesus Christ founded and established the Church for the administration of the Sacraments and the Holy Spirit is guiding and sanctifying the action of the Church towards the salvation of all the redeemed.And The Catholic Church expect every members to participates in the Sacraments in manner worthy of receiving the Real Presence of Jesus Christ thru the Holy Eucharist and expect the highest reverence in receiving this powerful Sacraments.

Godbless

How do I know when to cross myself during the Mass of Pope Paul VI (Novus Ordo)?

To start with, prior to Mass, we bless ourselves with Holy Water at the entrance of the church by dipping our fingers in into the Holy Water and making the sign of the cross on ourselves.

The same is done when exiting the church after Mass.

1. Dipping Your Hand in Holy Water

When you come into the church building, what is the first thing you do? Don't, you dip your hand into the holy water and make the sign of the cross? Why do you do that? Well, for three reasons: a. in repentance for your sins; b. for protection against the Evil One; c. to remind you of your baptism.

a. Holy water reminds us to be sorry for our sins. When there is the rite of sprinkling in the liturgy, we always sing the Asperges, which means "you will sprinkle or wash". Asperges me hysoppo et mundabor, lavabis me et super nivem dealbabor. These are words from the great penitential psalm, Ps 50: You will sprinkle me with hyssop and I shall be cleansed: you will wash me, and I shall be made whiter than snow.

b. Holy water is a sacramental which is a protection against the snares of the devil. The old prayer for the blessing of holy water said: "0 God, creator of unconquered power, King of invincible empire and victor ever-great: who put down the powers of hostile dominion and conquer the fury of the roaring enemy, who fight powerfully against our wicked foes: trembling we beseech you, 0 Lord, we implore you and beg you: that you might graciously look upon this creature of water and salt, kindly illumine it, sanctify it with the dew of your loving kindness, so that wherever it is sprinkled, through the invocation of your holy Name, every infestation of the unclean spirit be cast out, and the terror of the poisonous serpent be driven far away. And may the presence of the Holy Spirit deign to be with us always, we who implore your mercy."

c. Holy water reminds us of our baptism: of that great day when we ourselves or our parents or sponsors - renounced Satan, professed faith in Christ, and were baptized into the mystery of the Holy Trinity. At that moment all our sins were forgiven: original and actual, and we became children of God, filii in Filio, heirs of the promise, daring now to call God our Father.

When, you dip your hand into the holy water font, remember these things, and like Our Lady, treasure them in your heart. - Sacred Signs and Active Participation at Mass

During the actual Mass itself we make the sign of the cross at thee different times. All other times the faithful make the sign of the cross are not listed in the norms or guidelines of the Mass and are to be considered optional or due to personal piety either from a personal or regional custom.

When we make the sign of the cross during the actual Mass it is determined by both the rubrics of the Mass and personal tradition.

How many times are Catholics meant to do the sign of the cross during Mass?

1.The celebrating Priest opens the Mass with a sign of the cross, which the congregation joins in.

2.Just prior to the the Gospel reading there is an older version of the sign of the cross preformed by drawing out a small cross over the forehead, the lips, and the chest.

3.The celebrating Priest concludes the Mass with a sign of the cross, which the congregation joins in.

Optional times:

1.Each time you genuflect you can simultaneously make a sign of the cross (which should be done when you enter Mass, exit Mass, and cross in front of the Altar)

2.When the celebrating Priest passes by during the entering and exiting processions.

3.When a Priest gives a blessing at Mass.

4.After receiving the body and blood of Christ each participant preforms a personal sign on the cross.

5.When you feel moved to “put on the armor of God”

To this list of options, I would like to add that some priests start their homilies with the sign of the cross which the faithful are encouraged to do also.

In short there are several other times the faithful may make the sign of the cross out of personal piety.

For those who wish to peruse more information on the subject here is the GENERAL INSTRUCTION OF THE ROMAN MISSAL from the Vatican.