Going lucid means realizing that you're dreaming, while you're dreaming. Once you have this realization you can control your dream: you can fly, or confront your phobias and inner demons, or ask wise people about the meaning of life.
Lucid dreams are an important part of many spiritual traditions, from the dream yoga of Tibetan Buddhists to the shamanic practices of North and South American tribes. But they have also been rediscovered and practiced by many practical, left-brained westerners.
One of these was physicist Richard Feynman, who accidentally discovered lucid dreaming when he had to write a college essay on "Stream of Consciousness" and approached the problem like a true scientist:
"I got interested. Now I had to answer this question: How does the stream of consciousness end, when you go to sleep?
Once he had this realization, he was able to have many lucid dreams - sometimes making careful observations about his dream consciousness, sometimes just conjuring up lots of swimsuit models.
Decades later, Carlos Castaneda described the same technique in The Art of Dreaming, quoting don Juan Matus teaching him, "There are no steps to follow. One just intends to become aware of falling asleep."
Guayusa and Lucid Dreaming
What guayusa does is to prolong the hypnagogic (= falling-asleep) state, letting you carry your consciousness into your dreams.
The preferred method is to wake up in the middle of the night and drink a cup before falling back asleep. One skeptical person tried it, and described the result on YouTube:
"I was still thinking 'Why aren't I quite getting back off to sleep?' when I was on a whopping great fishing boat, fishing for whales..."
His verdict: "It was weird, but also quite interesting."
(Incidentally, this doesn't mean that guayusa is hallucinogenic. Some people can get the same effect with coffee or even food. Guayusa is just more efficient.)
The Stumbling Block
There is one BIG problem with lucid dreaming: it's less restful than regular sleep, so the brain naturally avoids it. Richard Feynman stopped going lucid in order to get more rest (he made the decision inside a dream!), and even Carlos Castaneda seems to have had trouble once the novelty wore off.
So your interest and intention have to be greater than your desire to get some Z's, or you'll miss lucidity and simply sleep.
Other Lucid Dream Techniques
There are other lucid dreaming techniques besides the one described here (of observing your consciousness while drifting off).
One method is to give yourself an intention beforehand. Carlos Castaneda describes this in chapter 10 of Journey to Ixtlan, where don Juan instructs him to focus on his hands to control his dreams. Lucid dream author Robert Waggoner describes his success with this method in an interview.
Another method is to perform reality checks. The problem with this method is that the mind can rationalize almost anything, just to stay asleep (the stumbling block, again).
And speaking of the somnovolent mind, Sigmund Freud came awfully close to finding his own method, as he describes in chapter 3 of The Interpretation of Dreams:
".... there is a dream that I can produce in myself as often as I like - experimentally, as it were. If I eat anchovies or olives or any other highly salted food in the evening, I develop thirst during the night which wakes me up. But my waking is preceded by a dream; and this always has the same content, namely, that I am drinking. I dream I am swallowing down water in great gulps, and it has the delicious taste that nothing can equal but a cool drink when one is parched with thirst."
If only he had recognized the thirst and drinking as a dream sign, he would have gone lucid... and possibly changed psychoanalysis forever, creating a post-neurotic world of shamans (instead of the monkey cage we live in now).
The Richard Feynman quote starts on page 47 of the (great) book, Surely You're Joking, Mr Feynman!.
Other YouTubers describe their experience taking guayusa for lucid dreams.
Tenzin Wangyal Rinpoche explains the goals of dream yoga: "The state of your mind as you fall asleep can determine the course of your dreams, and the course of your life."
For the dream instructions in Journey to Ixtlan, see chapter 10 (pp 97-100), chapter 11 (pp 111-113), chapter 12 (pp 131-132), chapter 13 (pp 152-153), and chapter 15 (pp 198-199).