According to St. Louis de Montfort, at the end of the 33-day preparation we should go to Confession and receive Holy Communion with the intention of giving ourselves to Jesus Christ as slaves of love, by the hands of Mary. After Communion, we should recite the consecration prayer; we ought to write it, or have it written, and sign it the same day the consecration is made.
See below for a link to the Act of Consecration prayer to be made on the Marian feast day.
More from St. Louis de Montfort:
It would be well that on this day, we should pay some tribute to Jesus Christ and our Blessed Lady, either as a penance for our past unfaithfulness to the vows of baptism, or as a testimony of dependence on the dominion of Jesus and Mary. This tribute should be one in accordance with your fervor, such as a fast, a mortification, an alms, or a candle. If but a pin is given in homage, and given with a good heart, it will be enough for Jesus, who loves only the good will.
Once a year at least, and on the same day, we should renew the consecration, observing the same practices during the three weeks. They might also once a month, or even once a day, renew all they have done, in these few words: "I am all Thine and all that I have belongs to Thee, O my sweet Jesus, through Mary, Thy holy Mother."