Soviet elections had a regular impact on everybody's lives and tens of millions were involved in campaigning, ensuring the population remained engaged and committed to the Soviet system. Essentially every adult could vote and it was effectively compulsory: if you hadn't voted by the afternoon of polling day, someone would remind you, bring you a ballot box, etc., until you did. If you didn't vote, you were making a point and it might well be followed up.

Candidates had to be nominated by either the CPSU or other social organizations (e.g. trade unions) controlled by the CPSU; citizen nomination was unconstitutional (this is the key point of dispute in the recent Hong Kong protests). In practice, only one candidate was nominated in each constituency; in the early years there was an elaborate process to achieve this, in later decades the key power-brokers in the area sorted it out in an informal meeting. Not all deputies (especially at local level) were CPSU members and a lot of effort went into getting a balanced slate, e.g. both men and women, different ethnic groups and especially different socioeconomic groups. For example, women and party members were around 40-45% of candidates throughout the thousands of different Soviets. That ensured (almost) everybody appeared to have a stake in the system.

The actual mechanism of voting was either to leave the ballot unmarked or to cross off the candidate's name. That is, the default option was to vote for the candidate; voting against the candidate required definite action (similar to the Mainland Chinese system where you put an X against a candidate you oppose). You had the option of voting secretly or openly, but all of the ballots were read afterwards. People could and did write comments on the ballot paper which were extensively analyzed afterwards as a kind of public opinion research.

Candidates had to achieve over 50% to win and every election candidates failed to achieve this. Obviously you had to be either exceptionally unpopular or incompetent to lose under these conditions and (I'm slightly going beyond the sources here) it was probably pretty bad for your career. Dissent was most common in Estonia and Latvia (which of course had recent experience of democratic government) and also in Kazakhstan (where many migrants were very unhappy with their conditions, without anyone questioning their loyalty to the system that brought them there).

Sources: http://www.jstor.org/stable/149653 and http://www.jstor.org/stable/150362