It's really a bit too complicated to go into here, but most of the standard texts on rigging will have it, with illustrations. Try "The Rigger's Apprentice" by Brion Toss, for a start.
Peter.
<A target="_blank" HREF=http://www.ybw.com/cgi-bin/forums/showthreaded.pl?Cat=&Board=cb&Number=236138&page=&view=&sb=&o=&vc=1#Post236138>Here's</A> a description from last year.