1.Check to make sure that OWA is working internally first, by going to a web broswer, type in
http://internalexchangeserveraddress then enter
Uername = domain\username
password = windows network password
2.You need to open an http port 80 or https port 443 (depending on your setup) in your firewall NAT that allows a passthrough from one of your external ip addresses to the internal IP of your exchange server. You might also have it add it to the rules of the firewall if it doesn't automatically add like some firewalls do.
3.Once this port is open, you should be able to type in the IP address from outside your network, and you should get a popup to type in username and password.