There are example files in /usr/share/examples/ipf/. You can also have a 
look at these webpages:
http://www.netbsd.org/Documentation/network/
http://radon.moof.ai.mit.edu/~armenb/ipnat.html
And remember: Have ip-forwarding switched on in the kernel! It's not men-
tioned in the docs. If you then follow the examples, it works fine.
Bye,
   Alex.