you can get a minimal NAT up and running with a few tweaks to the /etc/pf.anchors/com.apple file. Included is and example. The only modification you'll need to make is in the 900.NATrules file. Just change the external_interface and internale_interface to fit your environment.

you may also need to do this for testing

sudo sysctl -w net.inet.ip.forwarding=1 

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and edit you /etc/sysctl.conf file to include this for a more perminant setting

net.inet.ip.forwarding=1