Qualifications of guru and guru worship
While Srila Prabhupada was personally present, he appointed certain of his disciples to initiate on his behalf. He said that after he left this world, they could become regular gurus. This is the standard Vedic system. While one's spiritual master is still present, one shouldn't accept disciples of one's own, unless given permission by his guru. Srila Prabhupada wanted all his disciples to act as spiritual masters and to carry on the disciplic succession.
Worship is a Vedic custom that is not restricted to gurus. We're recommended to worship the Lord, the demigods, the guru, the brahmanas, and one's superiors. See Bhagavad-gita 17.14.
In ISKCON, any disciple of Srila Prabhupada in good standing can become a guru--that is, those who are steady in their vows to chant sixteen rounds (1728 times) the Hare Krishna maha-mantra daily and refrain from illicit sex, intoxication, meat eating, and gambling. Recommendations from responsible leaders are also required.
In the Vedic tradition there are two types of gurus, diksha-guru, the spiritual master who gives initiation, and shiksha-guru, the instructing spiritual master. Generally, one's shiksha-guru eventually becomes one's diksha-guru after some time.
To become recognized as a spiritual master in ISKCON, one currently has to be authorized by the GBC, Governing Body Commission, which is the highest managerial authority in ISKCON. To become a member of the GBC, one is recommended and selected by other members—generally a serious and respected devotee from a region. There are GBC representatives from each country. Their aim is to manage the society in accordance with Prabhupada's desire, which is to spread Krishna consciousness in a unified way.