As a leading rabbi, Nicodemus doubtless had disciples himself. When he sensed that Jesus was imparting something important, he was told that the way to learn would be to be baptized and then to learn more. A new birth would lead to a new process of learning. We gain insight into how Jesus and his apostles may have viewed the effectiveness of the rite of baptism from a note in the Fourth Gospel. This could have been owing to more pressing duties but may have been to avoid creating the impression that the baptizer or the baptism itself was the important thing, rather than the new life that was beginning and the new process of learning.
Baptism seems to have been performed rather quickly after conversion.
There was not an extensive examination of candidates or a long probation. The order was baptism, then instruction. This does not mean that baptisms were indiscriminate - repentance and faith were prerequisite. It simply underlines the point that those who were baptized were still only beginners. Eligibility for Baptism Jesus welcomed tax collectors and sinners into his circle.