Here are some of the details of the mitzvah to appoint judges, as expounded by the sages. [This continues the exposition of the Chinuch’s text at mitzvah 491.]
The first set of details addresses the seating arrangement of the seventy Sanhederin. The ablest among them would be seated below the Nasi (also known as the Rosh Yeshiva). He is given the title Av Beis Din. The remaining sixty-nine would be seated alongside the Nasi in a sequence reflecting their learnedness and their age. That is, the more learned someone was, the closer to the Nasi would be his position in the series. If two (or more) individuals were of equal rank as far as learnedness was concerned, they would be seated earlier or later in the sequence based on their age. They formed a semi-circle, so that each one would be visible to everyone else.
Apart from the main judiciary body, that is the Beis Din Hagadol, two additional judiciary bodies, comprising twenty-three judges each, would be stationed nearby. One would be situated at the entrance to the Azara; and the other would be situated at the entrance to the Mount of the Temple. The most learned individual of each of these bodies served as the Head of his respective body.
The only ones qualified to be appointed to any of the judiciary bodies (big or small) were individuals possessed of wisdom and deep understanding of Torah knowledge. They also had to possess some amount of knowledge of other areas of inquiry – including, for example, medicine, mathematics, astronomy, astrology, sorcery, and magic. They needed to be able to fall back on knowledge of these areas of expertise, in case circumstances demanded it. In addition, in order to qualify, a person – whether a kohein, a leivi, or a yisra-el – had to have privileged pedigree, sufficient to make his daughter eligible for betrothal to someone of the priestly lineage (kehuna). That it is so is derived from the verse vehisyatzvu sham imach, which is interpreted as implying that the judges that Moshe would appoint had to resemble him (in point of pedigree). [And so too for judges generally.]
Now, the only ones who could be appointed to a judiciary body, whether it be the Sandederei Gedola or the Sanhederei Ketana, were individuals who received ordination. Moshe our teacher performed ordination by hand upon his pupil Yehoshu-a – as it is written vayismoch yadav alav. He likewise performed ordination by hand upon the seventy elders whom he had assembled (to form the original Sanhederei Gedola). They, in turn, performed ordination upon others, who in their turn performed it on yet others…continuing a process that culminated in the last of the properly ordained. However, the ordinations performed subsequent to those conferred by Moshe differed from those of his in that they were not performed by hand. Instead, the issuers of ordination would carefully examine the candidate receiver of ordination. If he was found to be well versed in the knowledge of Torah; if, further, he proved himself to be well informed and of sound mind; and if, finally, he exemplified a set of personal traits that included love of truth and detestation of inequity in all its forms – then, provided that he glowingly passed all these tests of personality and intellectual ability, he would be pronounced ordained by three people who included in their number at least one who was himself ordained. Thenceforth, he would be designated with the appellation of rebbi (which is to say, master-teacher). He would, from that point onward, be empowered to adjudicate matters pertaining even to the imposition of fines.
There are numerous additional details that pertain to the qualification for appointment to judgeship. They are spelled out and discussed with due amplification in Tractate Sanhederin.