To rely upon the teachings of the two higher classes of Tantra. 

In general, our practice of Tantra usually has two phases.  In the first phase, we have not yet received a highest yoga tantra empowerment of Heruka and Vajrayogini.  During this phase, we will usually receive some lower tantra empowerments, such as Tara, Amitayus, Vajrasattva, and so forth.  During this phase, we can begin to have a daily practice of Tantra in the context of the corresponding deity sadhana.  Our main focus at this time should be to gain some initial experience of self-generation, and in particular we should start to consider deeply the relationship between our Lamrim practice and our Tantric practice.  The second phase of our Tantric practice begins after we have received the highest yoga tantra empowerments.

Once we receive our highest yoga tantra empowerments, we generally try follow Atisha’s advice to the Translator Richen Sangpo to integrate all our Deity practices into our practice of our personal Yidam by recognizing they are all the same nature and dissolving them into our Yidam (Yidam means personal deity, in this context it means Heruka or Vajrayogini).  Practically speaking, what this means is when we practice Heruka or Vajrayogini directly, we mentally understand we are practicing all of the other deities we have ever received empowerments into indirectly.  This is most easily accomplished by recalling that our Yidam is actually an emanation of our guru in the aspect of our Yidam, and our Guru is the synthesis of all the Buddhas.  By practicing in this way, every time we receive the empowerment of any deity, we seek to incorporate the essential meaning of the instructions we receive into our main practice of Heruka or Vajrayogini.  This does not mean we can’t practice the other deities directly.  We can of course do so when it feels like the right thing to do, but generally we deepen our practice of our principal deity.

The practice of Highest Yoga Tantra has two main stages, generation stage and completion stage.  Generation stage is when we change the basis of imputation of our I from our ordinary body and mind to the completely pure gross body, speech and mind of our Yidam.  Completion stage practice is when we change the basis of imputation of our I from the gross deity body to the completely pure subtle (and indeed very subtle) body, speech and mind of the deity.

For simplicity, we can view the entire path of Sutra and Tantra as gradually drawing our winds inward into our central channel at our heart.  We begin our training as an ordinary being.  We then meditate on the Lamrim and generate the sincere wish to become a Buddha.  Our identity has shifted to that of a Bodhisattva, and our inner energy winds have gathered into such a being.  Then we train in generation stage where we draw our identity and winds into the pure body, speech and mind of the Yidam.  Then we train in the body mandala meditations, which is like a bridge between generation stage and completion stage.  Then, we train in drawing our identity and winds into the different mantras of our Yidam.  The mantra of each deity of the body mandala is like the condensed meaning, or inner essence, of each deity.  Body mandala meditations and recitation of mantras function to purify the gross levels of our deity’s subtle body, and draw our identity and winds into it.  Then we begin completion stage.  First we train in bringing our identity and winds into our central channel at our heart.  Then into our indestructible drop at our heart.  Then into the seed letter of the deity inside the indestructible drop.  This seed letter is like the condensed meaning of all of the deities of the body mandala.  It is also the same nature as our very subtle body.  This body never dies and it travels from life to life.  If we can center ourselves within it, we will become a deathless being.  Then we seek to draw into our very subtle mind of great bliss inside the seed letter.  Then we seek to draw ourselves into the emptiness of our very subtle mind of great bliss.  Finally, we seek to realize the non-duality of our very subtle mind of great bliss and the emptiness of that mind.  When we have realized this directly, we attain the realization of meaning clear light and we are very close to enlightenment.  In dependence upon the realization of meaning clear light, we purify our very subtle mind of all past delusions and contaminated karmic imprints.  When we have completely done so, we become a Buddha.

All of these practices are explained in detail in Guide to Dakini Land, Essence of Vajrayana, Tantric Grounds and Paths, Clear Light of Bliss, Mahamudra Tantra and Modern Buddhism.  When we have even a glimmer of an appreciation of the power of these practices, then we will effortlessly realize there is nothing more important we can do with our life than train in these two tantric stages.