To expand upon Pjen’s answer, if a geographical grouping submits a slate with as many countries as it has slots on the UNHRC, there is no vote. The slate of countries is accepted by acclimation. If there are more countries on a slate than slots for commission members, then there is a vote; the top vote getters get seats. At no point is there any review of a country’s human rights record.
In the past, human rights abusers have basically gamed the system so that groups like the G-77 (the poorest countries in the world, which generally have low regard for human rights) will only nominate countries that will shield them from being accused of human rights abuses. China scratches cuba’s back, which scratches Sudan’s, which scratches Zimbabwe’s, which scratches China’s… and so on.
Kofi Annan has proposed radical changes to the UNHRC, replacing it with a council that has members that are held to strict standards. Cite. Which is pretty much what I proposed last year, to which some folks were shocked and offended that the members of a UN human rights commission would be held accountable to some sort of standards for respecting human rights.