There ARE examples:
Linux IP NAT Forum
IP NAT configuration (Karl S. Hagen)
Date: Mon, 22 Jun 1998 23:46:19 GMT
From: Michael Hasenstein <firstname.lastname@example.org>
See my document (top of forum homepage) about NAT. Somewhere in the last 3rd there are various examples which should help you a lot.
Ok... Explain this.. :) by Karl S. Hagen, 7/06/98