ha ha! you might regret that question...
short answer: all the time!
when i was protestant, i thought catholics made the sign of the cross a lot, and wondered why.
when i became coptic orthodox, i wondered why the catholics hardly ever made the sign of the cross in the liturgy, and it was the same catholic church i used to attend
we like it as it's part of our participation in the liturgy, we are asking for a blessing, or repenting of our sin, or praising God, or trying not to let our concentration wander, or some combination of the above.
long answer:
(you have been warned)...
at the end of saying 'our father'
every time the priest or (sub)deacons or congregation say 'in the name of the Father, the Son and the Holy Spirit' (this happens quite at lot!)
every time anyone says 'blessed be...' (God)
any time anyone says 'worship' (e.g. the nicene creed when we say we 'worship and glorify' the Holy Spirit)
so, also any time we say 'glorify'
when we are praying as the bread for consecration is chosen, saying 'Kyrie eleison' (Lord have mercy) 41 times (the pious do one sign of the cross for every 'Kyrie eleison')
after the Bible has been read (optional)
before the gospel is read
any time anyone says 'Holy' (Agios) when referring to God
any time the priest waves incense vaguely in your direction (the priest censes the congregation and prays for them during the 2 epistle readings and during morning or evening prayers and other times as well)
any time the priest says 'peace be with you' while making the sign of the cross (optional, you can also bow you head in reverence at this point without making the sign of the cross, same as with the incense)
during prayers and intercessions (optional)
when the priest is telling how Jesus Christ took the Body/Blood and 'gave thanks', 'blessed it' and 'sanctified it'
before taking Holy Communion
after praying the prayer from the prayer book for 'after Holy Communion'
anytime the priest is blessing the congregation (optional)
and any other time during the liturgy that it seems appropriate to make the sign of the cross.
i may have missed a few, feel free to correct me...
Actually there's a fair bit of variation from parish to parish on some of these...
My priest instructed me that we should not make the sign of the cross when coming forward to receive (or rather, that it was not strictly necessary as some were insisting) since we are coming to receive the Body of the Incarnate Logos, so it is not necessary or necessarily fitting to bless ourselves in the Name of the Trinity at this time.
I was instructed not to make the sign of the Cross when the priest turns to bless the congregation, just to bow and receive the blessing. The priest is blessing us, and it is presumptuous to think to bless our selves. Later in the Liturgy, after the consecration, the priest no longer makes the sign of the cross, he just steps aside when he says "peace be with you", for the Lord is now present on the Altar, so the preist does not dare to bless in His presence. How much less should we think to bless ourselves when the preist does not even dare bless?
Similarly, if someone is giving a Sunday School lesson, they start "In the Name of the Father and the Son and the Holy Spirit", but if a priest is present the do not say this, they ask the priest to and then they begin. If a reader give the sermon, it is still the priest who says this. In any case those present cross themselves.
Also, when the preist censes the people I have been taught to bow only, and to be mindful of when the preist is actually censing the people, and when he is merely censing towards the west.
I have never seen the practise of crossing at "worship" and "glorify". I have only seen everyone bowing down in worship whenever reference to worship of God is made (and often when the Bible reading makes reference to worshipping Satan if people are half asleep and just hear the word without noticing the context...)
When the priest says "gave thanks", "blessed," "sanctified" during the institution narrative, I do not think we should cross ourselves. To each of these the deacon replies "Amen". Often people in the congregation say the "Amen" as well, but it is clear that it is not correct to do so (just as it is not correct to sing along with the priest's parts). Similarly, it is the role of the priest to sign the gifts. While it is not the preist alone who performs the Liturgy, rather all the people lead by the priest, still each has their role. At this time the priest signs, the deacon replies Amen, and the people participate in prayer in their hearts.
Many times "Holy, holy, holy" is said in quick succession, and you can see everyones hands whipping wildly across their chests, or making very small quick signs of the cross that one monk referred to as "playing their ukuleles". I do not believe that the Liturgy ever encourages to make the sing of the cross irreverently. I believe that in these cases, we should make the sign of the Cross once, slowly and prayerfully as we say "Holy, Holy, Holy", rather than three times quickly. It is one Trinity who is "holy, holy, holy", not three.
what I am familiar with is:
-At the conclusion of the Lord's prayer
-At the conclusion of, or optionally thought a set of "Lord have mercy" (41, 100, or 400).
-When "Blessed be He who comes in the Name of the Lord" is said.
-Whenever the Trinity is mentioned (e.g. "Blessed art Thou in truth with Thy Good Father and the Holy Spirit", or "In the Name of the Father and the Son and the Holy Spirit, one God, Amen.")
-When Holy is said in reference to God.
-After prostrating when entering the Church.
-optionally after prostrating (or worshipping by bowing on Sundays and during Pentecost when prostrations are forbidden), though I believe more in the context of a prostration (quick down and up) rather than a kneeling (prolonged).
(Of course it is much more important to focus in praying in spirit and truth rather than to focus too much on when the proper times to cross one's self are, and I don't mean to be argumentative.)
