What is the Catholic doctrine on how to humble oneself before the Lord?

An excellent Catholic work on humility is Fr. Cajetan Mary da Bergamo's Humility of Heart.
^{(A fully OCRed version available here.)}

The Rule of St Benedict of Nursia lists 12 degrees of humility. Although his rule was written for monks of the sixth century, there are many insights of great value for practicing humility for the average Catholic.

From the Holy Rule of St. Benedict, chapter VII. On Humility

The first degree of humility, then, is that a person keep the fear of God before his eyes and beware of ever forgetting it. Let him be ever mindful of all that God has commanded; let his thoughts constantly recur to the hell-fire which will burn for their sins those who despise God, and to the life everlasting which is prepared for those who fear him. […] Let a man consider that God is always look at him from heaven, that his actions are everywhere visible to the divine eyes and are constantly being reported to God by the Angels.

The second degree of humility is that a person love not his own will nor take pleasure in satisfying his desires, but model his actions on the saying of the Lord, I have come not to do my own will, but the will of him who sent me.

The third degree of humility is that a person for love of God submit himself to his Superior in all obedience, imitating the Lord, of whom the Apostle says, He became obedient even unto death.

The fourth degree of humility is that he hold fast to patience with a silent mind when in this obedience he meets with difficulties and contradictions and even any kind of injustice, enduring all without growing weary or running away.

The fifth degree of humility is that he hid from his Abbot none of the evil thoughts that enter his heart or the sins committed in secret, but that he humbly confess them.

The sixth degree of humility is that a monk be content with the poorest and worst of everything, and that in every occupation assigned him, he consider himself a bad and worthless workman.

The seventh degree of humility is that he consider himself lower and of less account than anyone else, and this not only in verbal protestation but also with the most heartfelt inner conviction.

The eighth degree of humility is that a monk do nothing except what is commended by the common Rule of the monastery and the example of the elders.

The ninth degree of humility is that a monk restrain his tongue and keep silence, not speaking until he is questioned.

The tenth degree of humility is that he be not ready and quick to laugh.

The eleventh degree of humility is that when a monk speaks he do so gently and without laughter, humbly and seriously, in few and sensible words, and the he be not noisy in his speech.

The twelfth degree of humility is that a monk not only have humility in his heart but also by his very appearance make it always manifest to those who see him. That is to say that whether he is at the Work of God, in the oratory, in the monastery, in the garden, on the road, in the fields or anywhere else, and whether sitting, walking or standing, he should always have his head bowed and his eyes toward the ground.

St Thomas Aquinas also writes about the 12 degrees of humility, but in the reverse order.

It would seem that the twelve degrees of humility that are set down in the Rule of the Blessed Benedict [St. Thomas gives these degrees in the reverse order to that followed by St. Benedict] are unfittingly distinguished. The first is to be "humble not only in heart, but also to show it in one's very person, one's eyes fixed on the ground"; the second is "to speak few and sensible words, and not to be loud of voice"; the third is "not to be easily moved, and disposed to laughter"; the fourth is "to maintain silence until one is asked"; the fifth is "to do nothing but to what one is exhorted by the common rule of the monastery"; the sixth is "to believe and acknowledge oneself viler than all"; the seventh is "to think oneself worthless and unprofitable for all purposes"; the eighth is "to confess one's sin"; the ninth is "to embrace patience by obeying under difficult and contrary circumstances"; the tenth is "to subject oneself to a superior"; the eleventh is "not to delight in fulfilling one's own desires"; the twelfth is "to fear God and to be always mindful of everything that God has commanded." For among these there are some things pertaining to the other virtues, such as obedience and patience. Again there are some that seem to involve a false opinion--and this is inconsistent with any virtue--namely to declare oneself more despicable than all men, and to confess and believe oneself to be in all ways worthless and unprofitable. Therefore these are unfittingly placed among the degrees of humility.

Let us not forget that those who desire to live a life of humility should also pray for this virtue. The following Litany of Humility was composed by Rafael Cardinal Merry del Val (1865-1930),Secretary of State for Pope Saint Pius X:

O Jesus! meek and humble of heart, Hear me. From the desire of being esteemed, Deliver me, Jesus.

From the desire of being loved... From the desire of being extolled ... From the desire of being honored ... From the desire of being praised ... From the desire of being preferred to others... From the desire of being consulted ... From the desire of being approved ... From the fear of being humiliated ... From the fear of being despised... From the fear of suffering rebukes ... From the fear of being calumniated ... From the fear of being forgotten ... From the fear of being ridiculed ... From the fear of being wronged ... From the fear of being suspected

That others may be loved more than I, Jesus, grant me the grace to desire it.

That others may be esteemed more than I ... That, in the opinion of the world, others may increase and I may decrease ... That others may be chosen and I set aside ... That others may be praised and I unnoticed ... That others may be preferred to me in everything... That others may become holier than I, provided that I may become as holy as I should…