Of course the epitome of humility is our Lord himself, coming "down" into this world and existence, as a human, born in the same way as all humans, living and dying like we all do, though the death of a cross.
Advent also includes a necessary humbling taking place by those who participate in this salvation event. Think of Mary and Joseph. Think of the shepherds and the Magi. Think also of those who refused to humble themselves, like Herod.
In humility Jesus came, and only in humility can we participate in this coming and gift. This will involve prayer on our part to God, acknowledging the truth about ourselves, our sin, and looking to him not only for forgiveness, but for cleansing and change. Humbling ourselves before God needs to be done regularly, especially when we know what is in our hearts is sinful.
When we're mired in our own sins, we have no time or heart for Jesus and God's work in Jesus in our lives and through the community of God in Jesus. Humility starts with the dawning on us that we are wrong and need God's help to be made right with a completely new start. While there needs to be those special "conversions" or clean breaks from a given sin issue- and this is important, at the same time there needs to be smaller conversions along the way. Only then can we be open to believe then see and participate in this great salvation Jesus brought into the world. And begin to learn to live, no less than in the very humility of our Lord himself in this world- together with others in Jesus and for the world. That others may come in faith and see and find this new life.