The Sign of the Cross means, essentially, “Amen,” indicating personal assent. The cross is Christ’s victory sign, and Christians have long used it to claim identification with his triumph over sin and death.
To make the sign, one touches fingertips to forehead, abdomen, left breast, and right breast. Traditional times are: after dipping fingers into the blessed water upon entering a church; upon concluding private prayer, before and/or after receiving the elements of Communion; during worship, at the Opening Acclamation, end of the Gloria in excelsis, end of the Creed, at “blessed” in the Benedictus est, Domine which is attached to the end of the Sanctus, and in the text of the Eucharistic prayer, and during the blessing concluding the Eucharist. In other words, watch other participants, and follow their lead. You catch on quickly.
At the Gospel, a special version of the sign of the cross is used, tracing the cross with thumb on one’s forehead, lips, and heart. This represents the ancient prayer, “God, be in my head and in my thinking; God be in my lips and in my speaking; God be in my heart and in my understanding.”