The oldest Christian gesture, and the shortest creed.
Forehead, breast, left shoulder, right shoulder, with the right hand, saying: in the name of the Father, and of the Son, and of the Holy Spirit.
Eastern Catholics go right shoulder to left, and join thumb and first two fingers for the Trinity. Both are correct.
A small cross is traced on forehead, lips, and heart before the Gospel: on my mind, on my lips, on my heart.
It contains both central mysteries of the faith in two seconds: the Trinity in the words, the Cross in the motion. Beginning and ending prayer with it puts everything between under those two facts.
Tertullian, writing around the year 200, describes Christians tracing the cross on their foreheads at every ordinary act, coming and going, dressing, and sitting down to eat.
The large sign across the body developed later, and the Trinitarian words attached to it as the Church defended the doctrine of the Trinity.
The gesture is part of the Church's common life and opens every liturgy. Made quickly and thoughtlessly it becomes a tic, which is worth resisting.