A person looking around on a Sunday morning may notice that different people cross themselves at different times. To a certain extent, when to cross oneself is a matter of personal piety and not of dogma. However, there are times in the service when crossing oneself (thumb and first two fingers touching each other, third and fourth fingers folded into the palm: touching head first, to stomach, right shoulder to left) is called for:
To cross: when you hear one of the variations of the phrase “Father, Son, and Holy Spirit”; before venerating an icon, Gospel, or Cross; when blessed with an icon, Cross, Gospel, or Chalice; entering and exiting the temple; when passing before the Altar.
Not to Cross: (only bowing of the head): when blessed with hand (as in “Peace be unto all”), or censed. In receiving a blessing from a bishop or priest one does not make the sign of the Cross beforehand. “In this way ought we to distinguish between reverence toward holy things and toward persons”.