A vassal would swear allegiance and pay homage to his lord in a commendation ceremony. The commendation ceremony was designed to create a lasting bond between a vassal and his lord. Fealty and homage are a key element of feudalism. The ceremony consisted of swearing the Knights Oath of Fealty and the act of homage. 

Knights Oath of Fealty - the Words
The promise of faithful service to the lord was called the Oath of Fealty. To take an oath was a very solemn proceeding; it was an appeal to God, by which a man called down on himself divine punishment if he swore falsely. The words spoken in the Knights Oath of Fealty were similar to the following:

"I promise on my faith that I will in the future be faithful to the lord, never cause him harm and will observe my homage to him completely against all persons in good faith and without deceit."

Knights Oath of Fealty - The Ceremony and the Act of Homage
The Knights Oath of Fealty was sworn during a solemn ceremony necessitating an act of homage. The vassal would appear before the lord bareheaded and without and weapons. The vassal would then kneel before the lord, clasping his hands as in prayer which he would stretch outward towards his lord. This position signified total submission. The vassal then swore the Knights Oath of Fealty. The lord would then take the hands of the vassal and announce his acceptance.