The Ngondro teachings are practiced in stages, as are all teachings of the dharma.  Each stage builds upon the understanding gained from studying, contemplating, meditating and applying the teachings that were taught previously.  Without having taken the time to integrate these teachings, the dharma becomes something of a conversation piece rather than something that has visceral power to change one's life.  Ngondro, which can be translated as foundation or as preliminaries,  although some commentators caution that understanding Ngondro as preliminary practices is misleading as it might suggest that  they have lesser value than later practices.  This is universally considered a mistake: His Holiness Dudjom Rinpoche says:  

Therefore, for the ultimate truth of the Great Perfection to appear in your mind depends upon the preliminary practices.  This is what Drikungpa meant when he said:  "Other spiritual teachings regard the main practice as being profound.  We regard the preliminary practices as being profound."

It is just as he said.

    However I do think it is useful to think of the Ngondro as those practices that come before in the same way that the foundation must be built before erecting the building.  And as the translators of Words of My Perfect Teacher have elected to use "Preliminaries", that suites me as well.

    Accordingly, Patrul Rinpoche presents the Ngondro in two stages:  the Outer Preliminaries and the Inner Preliminaries.  The Outer preliminaries include:

1. The preciousness of a human birth.
2. Impermanence
3. The Principle of Causality.
4. The suffering of cyclic existence.

    Without understanding well these four principles, one will not really be able to set out on the path free from suffering.  It is said that the thing that makes one a Buddhist, regardless of which tradition one practices or where one has been born, is the taking of Refuge in the Buddha, the dharma and the sangha.  Refuge is shared by Burmese Theravada, Chinese Chan, Japanese Nichiren and Tibetan Vajrayana.   And yet one will not take refuge authentically without fully understanding the four thoughts that turn the mind to the dharma, the four thoughts one begins the Ngondro with in the outer preliminaries.