While usually thought of as a reincarnation of a past master, it is apparently possible to become a Tulku for the "first time" in the current lifetime, based on the aspiration to reincarnate in the future, among several other requirements.
https://palisuttas.wordpress.com/2013/0 ... e-a-tulku/
...according to Dr. Berzin, tulkus are a relatively minor category of spiritual practitioner, certainly less than a lama – an overused title in the West – according to Dr. Berzin’s exposition, which is based on an impeccable mastery of the Tibetan tradition (Dr. Berzin is a master scholar-translator who has translated for HH the Dalai Lama and other high lamas and is a Buddhist practitioner in his own right). According to Dr. Berzin, one can even choose to become a tulku by fulfilling just four requirements.
First, the aspiring tulku must have strong bodhicitta, i.e., striving towards awakening and compassion for the benefit of all sentient beings. In Dr. Berzin’s words, this pure intention must be “highly developed,” but by no means so much so that it becomes “uncontrived,” i.e., automatic and operative in all situations and circumstances, even when asleep.
Second, and most importantly, the aspirant must have strong development of the generation stage of Anuttarayoga Tantra (see also Deity Yoga), but by no means must have completed (perfected) this stage. The generation stage involves self-identification with a yidam, or enlightened being, including “visualizations for the transformation of death, bardo, and rebirth,” what we might refer to more simply as the death-rebirth experience, or conscious dying. This attainment is certainly not insignificant, but on the other hand it is by no means the highest attainment either.
Third, he or she must make continuous and strong prayers for continuing as a tulku into their next rebirth.
Finally, fourth, they must have followers who are able and willing to look for and find them after they have passed away.