Note: If you don't have blessed chalk, buy an inexpensive pack of chalk and ask your priest to properly bless it.
Write with blessed chalk above the door like so:
20 + G + M + B + 12
O Lord, almighty God, bless this house that it may become a shelter of health, chastity, self-conquest, humility. goodness, mildness, obedience to the Commandments, and thanksgiving to God, Father, Son and Holy Ghost. Upon this house and those who dwell herein may Thy blessing remain forever. Through Christ our Lord. Amen.
Epiphany Chalk is used to write the inscription over all the doors of one’s house. Soft classroom chalk of any color is best for writing on varnished or painted surfaces. A priest should bless the chalk on Epiphany Day, before the celebration of the main Mass of the day, using the following Blessing, which is taken from the Roman Ritual:
V. Our help is in the Name of the Lord.
R. Who made heaven and earth.
V. The Lord be with you.
R. And with thy spirit.
Let Us Pray:
O Lord God, bless + this chalk which Thou hast created, that it may be helpful to mankind; and grant that through the invocation of Thy most holy Name all those who use this chalk or with it write over the doors of their houses the names of Thy Saints, Gasper, Melchior, and Balthazar, may by their merits and intercession receive health of body and protection of soul. Through Christ Our Lord. Amen. (Priest now sprinkles chalk with Holy Water.)
Later that day – or any day within the Octave of the Feast of the Epiphany – at home, the ceremony is completed by a priest, or the father, or senior member, of the family. The following prayer is said once and at the main entrance of the house. All the family as he or she goes from door to door – everyone keeping a respectful silence, or better yet, reciting the Joyful Mysteries of the Rosary.
Let Us Pray:
Hear us, O Holy Lord, Father Almighty, Eternal God, and send Thy Holy Angel from Heaven to watch over, cherish, protect, be with, and defend all who live in my house. I call upon Your Saints, Gasper, Melchior, and Balthazar, to protect my family and my home from every harm and danger, and I place the marks of their holy names over the doors of my home to remain there as a constant reminder to us and to all who enter here that my house is truly a house of the Lord.
O God, make the door of my house wide enough to receive all who need human love and companionship; narrow enough to shut out all envy, pride, and strife. Make its threshold smooth enough to be no stumbling clock to children, nor to straying feet, but rugged and strong enough to turn back the temper’s power.
O God, make the door of my house the gateway to Thy eternal Kingdom. I ask these things in the Name of Jesus Christ Thy Son. Amen.
Note: Another version of this blessing, as well as a Blessing of Gold, Incense, & Myrrh (the gifts given by the wisemen) can be found in the Rituale Romanum online version.