The chalking of the doors is a centuries-old practice throughout the world. While less known in the United States, it is becoming more celebrated and an easy tradition to adopt, dedicating our year to God, asking His blessing on our homes and on all who live, work, or visit us there. The Feast of the Epiphany is traditionally celebrated on January 6th, or on the Sunday between January 2 to 8th.

The leader should be the head of the household (father, if the father is present). A reader may also be chosen. If one lives alone, the responses are also done by the leader himself, or one might invite friends and neighbors for the celebration. If the home has put out a Christmas crèche, that might be the best place to gather. The letters C, M, and B stand for the traditional names of the “three wise men.”

The leader makes the sign of the cross:
“+ In the name of the Father, and of the Son, and of the Holy Spirit.” All reply, “Amen.”

The leader greets those present in the following words:
Let us praise God, who fills our hearts and homes with peace. Blessed be God forever.
Response: Blessed be God forever.

The Word became flesh and made his dwelling place among us. It is Christ who enlightens our hearts and homes with his love. May all who enter this home find Christ’s light and love.

Listen to the words of the holy Gospel according to Matthew:
When Jesus was born in Bethlehem of Judea, in the days of King Herod,
behold, magi from the east arrived in Jerusalem, saying,
“Where is the newborn king of the Jews? We saw his star at its rising and have come to do him homage.”
When King Herod heard this, he was greatly troubled, and all Jerusalem with him.
Assembling all the chief priests and the scribes of the people,
He inquired of them where the Christ was to be born.
They said to him, “In Bethlehem of Judea, for thus it has been written through the prophet:
And you, Bethlehem, land of Judah, are by no means least among the rulers of Judah;
since from you shall come a ruler, who is to shepherd my people Israel.

Then Herod called the magi secretly and ascertained from them the time of the star’s appearance. He sent them to Bethlehem and said,
“Go and search diligently for the child. When you have found him, bring me word, that I too may go and do him homage.”
After their audience with the king they set out.
And behold, the star that they had seen at its rising preceded them, until it came and stopped over the place where the child was. They were overjoyed at seeing the star,
and on entering the house they saw the child with Mary his mother.
They prostrated themselves and did him homage.
Then they opened their treasures and offered him gifts of gold, frankincense, and myrrh.
And having been warned in a dream not to return to Herod,
they departed for their country by another way."

The intercessions are then said:
Leader: The Son of God made his home among us. With thanks and praise let us call upon him.
Response: Stay with us, Lord.

Reader: Lord Jesus Christ, with Mary and Joseph you formed the Holy Family: remain in our home, that we may know you as our guest and honor you as our Head. We pray:
Response: Stay with us, Lord.

Reader: Lord Jesus Christ, you had no place to lay your head, but in the spirit of poverty accepted the hospitality of your friends: grant that through our help the homeless may obtain proper housing. We pray:
Response: Stay with us, Lord.

Reader: Lord Jesus Christ, the three kings presented their gifts to you in praise and adoration: grant that those living in this house may use their talents and abilities to your greater glory. We pray:
Response: Stay with us, Lord.

The Leader then prays the blessing: Lord God of heaven and earth, you revealed your only-begotten Son to every nation by the guidance of a star.
Bless this house and all who inhabit it. Fill us with the light of Christ, that our concern for others may reflect your love. We ask this through Christ our Lord.
Response: Amen.

The leader concludes the rite by signing himself or herself with the sign of the cross and saying: May Christ Jesus dwell with us, + keep us from all harm, and make us one in mind and heart, now and forever. Response: Amen.

The leader takes the blessed chalk and marks the lintel (the doorframe above the door) on the inside of the main entrance to the house as follows: 20 + C + M + B + 24
(insert the last two digits of the current year)

Other doors may be chalked, for example, the bedroom doors by those who sleep in each room. It is preferable to end the celebration with a suitable song for example, “O Come, All Ye Faithful” or “We Three Kings.” Afterwards the family or host and invited guests might share a meal or a twelfth-night party, or enjoy a king cake.

—Adapted from "Prayers for Days and Seasons" USCCB

ND: The prayer the priest needs to bless chalk can be found in the Rituale Romanum just search the PDF for the word "chalk"