Be actively involved with your breed for many years, work you way up through the judging lists from C to A. Keep a record of the shows classes and numbers of dogs judged. When the figures get into the realms of several hundreds, then maybe you might be asked to give 'tickets' but the KC must aprove you.
Also you need to attend breed specific seminars, do some ring stewarding and also attend seminars on movement and conformation and rules and regulations.
Just remembered something else. To progress a judge needs to have demonstrated a fairly high level of showring success, by having bred or owned champions and/or other stud book number holders.