Reviewers (we call them editors) all start out the same: they click on the "Become an Editor" link and submit an editor application.
There is an entire forum here dedicated to that process, so I won't bore you with the details.
All editors start out small, with the vast majority beginning in categories with fewer than 50 entries. The editors work that category, turnign the theoretical guidelines into practical guidelines. They read, the participate in the internal forums, they learn how to hunt for sites, spot spam and other forms of directory abuse and at some point want more.
The editors then go through a process that is very similar to the initial editor application process. They ask for category "X", which should be a little bigger, but not significantly so, identify sample URLs and wait. If they are granted the permission, the process begins anew. If they are turned down, they go back to the learning, gain more experience and submit another request somewhere down the road.
That, is an overly simplified description of the process, but it is reasonably accurate without bogging you down in details.
New editor applications are only reviewed by what we call Meta Editors, who are the senior most editors in the project and have the richest experience and best judgement as to what it takes to become a good editor.