Tantra is one of the most misunderstood (and abused) of spiritual concepts expropriated from India.
Tantra in its broadest sense simply means technic. In particular ritualistic esoteric spiritual technics meant to activate or tap into ones spiritual energies.
Both in India and the West it has come to be abused. In India today (in particular in pop-culture, as in movies) it has come to be almost always associated with black magic, and in the West it is almost always associated with sexual rites.
If you want to know about some serious tantra, you can gleam a lot from the Aghoris, Siddhars, Samanars, Rishis, Nagas, etc. of India. My favorite source of inspiration are the Aghoris of Kerala and the Siddhars of Tamil Nadu. One of the best known works of the later, in the Saiva Siddhanta school of thought, is the Thirumanthiram (திருமந்திரம்) composed in 10th century BCE by Thirumoolar (திருமூலர்).
The Tantric Path
Tantra (i.e. “tantric rituals”) is a very personalized and private affair. Again, you need only observe the lives of the Aghoris, Siddhars, etc. of India. Even with a guru you don’t learn or follow a tantra from him, but you seek/develop your own, often starting with broad base of existing tantras and then crafting, fine tuning, and refining it to your spiritual growth. This requires lot of rigor and critical thinking (the kind of insight you get through meditation).
Serious tantra (i.e. those who take up tantra exclusively as their chosen path) is a very rigorous and direct path, and all the more dangerous. It is a walk on the razors edge, full of traps and enticements, that can set you back to square one, or worse. If the practitioner has even the smallest vestigial remains of any vices, then he will be tempted by “the power and magic” he gains from tantra, and deviate from his original goal (of self-realization), sending him in the wrong direction, into a dark alley which will be more difficult to break away from than the bonds of this world (as the more the power and magic he entertains, the more stronger the bonds), or gets locked into a hysteresis loop and can develop all sort of psychological conditions (neurosis, schizophrenia, delusions, etc) as he tries to “bridge” two different worlds.
This is why the pure tantric path is called the left-hand path of God (in particular the more demanding of all tantric paths – that of the Aghoris). This is because it uses technology (tantra) as props or as a catalyst to awaken your kundalini. As with any technology, there is a chance that technology can outpace the rate of our human evolution, i.e. it becomes more than we can handle (as it is happening the world today). Other paths in spirituality also use tantra but only as a minor or supplemental catalyst, and the only props come from your mind, heart, and body (i.e. not external) nurtured by yoga, asanas, puranas, bhakti, critical thinking, meditation, puja, etc.
The bhakti saint, Mata Amritanandamayi said (heavily paraphrased by me, from what I can remember) that for most people the kundalini is like a lotus in its unopened/bud stage. The best way to make a flower bud to bloom is to let it evolve at its own pace, and not by prying or forcing open the petals (in which case you’ll damage it). She also said, the progress of India [or for that matter spirituality] is about evolution and not revolution.
Thus as a minimal preparation of the mind before taking up the venture, one must put into practice the eight generalized rules (yama, niyama, asana, pranayama, pratyahara, dharana, dhyana and samadhi) outlined in the Yogas Sutras of Patanjali. In particular the yama and niyamas being an absolute must. While others can be flexible on these rules, those who take up tantra exclusively cannot afford to be lax on it.
Almost every Hindu practices tantra on a daily basis, in the form of their puja rituals (whether it be at home or at a temple). Though puja touches tantra only at the surface level, it more accurately reflects what tantra is than the way it is understood in the West. If anyone in the West wants to know about tantra, then they need to just visit a temple and observe all the elements of a typical puja: mantras (vibrations), yantras (symbols/devices), lamps, incense, camphor, flowers, water, five-metal vessels, alchemy, sacred herbs, offerings (often fruits and cooked grains), and a rich array of images (of God), etc. Take all these together, and practiced as a punctually without fail, it helps focus the mind, detach from the senses, prepares you for meditation, elevate you into higher consciousness, awakens your kundalini.