In trials the People’s Court is presided over by a judge assisted by two jurors who actually have most of the rights and responsibilities of the judge, and might, therefore, be called co-judges. The judges for the People’s Courts are elected for a one-year term by the local Soviet and may be recalled either by the same body or by the Commissar of Justice for sufficient cause. This provision for impeachability is one of the basic principles of organization of the Soviet courts.
   The judges of these lower courts are representatives of the working class for they themselves are of their number. A judge (and, he seemed an adequate one), in a city court of Moscow with whom the writer talked was a baker before he took up his present profession. The qualifications of the office are that in addition to being from among the toilers the judge must have “a record of two years’ responsible work in state or workers’ and peasants’ public trade union, or party organization of the workers, or of three years’ practical work in organs of Soviet justice in the capacity of not less than judicial investigators.” Whether this may be regarded as high fitness or not, it seems to insure the selection of men of fair ability for the job. Perhaps it is that business of recall that makes them so careful, but those whom the writer saw sitting were impressive in their earnestness and efforts to arrive at a constructive sentence.
   The jurors, whom we may think of as co-judges, are about 40 per cent. women and 60 per cent. men. These persons are elected by factory committees, Red Army sections, and village Soviets, and a special commission distributes the number to be elected among these various groups in the percentage of 50 per cent. to the first, 15 per cent. to the second, and 35 per cent. to the third. There are no specific qualifications, except the right to vote, but there is the restriction that “A person has no right to be a juror, if he has been expelled from a social or professional organization for a disgraceful offense or conduct, for a period of three years from the day of expulsion. . . .”
   The names of the candidates for jurors are posted, and objections, if motivated, may be presented to the election committee. When the list of elected ones is finally sent to the commission, the latter prepares another list of those to serve in the various courts. Each serves for only six days of the year, and during absence from work in the act of performing this duty the worker retains his place and wage of employment.
   While there seems to be no specific preparation for the task of a juror beyond the requirement that the judge explain to those who sit with him their rights and duties, there are conferences held and some evening classes which acquaint them better with the work before them.

From Chapter V of Russian Justice (published 1935)

Should you wish to dive deeper into this, besides the above book, there is—I have not read it—also the book Soviet Administration of Criminal Law by Judah Zelitch, published 1931. It is a much more academic book (the work done originally for a thesis), which describes the development of the law and courts from the revolution until 1931.