We just upgraded our work firewalls from Watchguards to Palo Alto, and our watchguards were very old units. So, I don't know how they compare to the new watchguards.
But, with ours, what you have to do is configure your network addresses for both WAN and LAN, and then you go into policy manager and create a policy for traffic through the firewall.
If you just want basic access, you just need http and https policies. Of course, you will need much more than that to allow all other traffic access, but that will get you started.
Again, I know it's not much to go on, but without knowing the new watchguard devices I would just be spitting into the wind trying to be more specific.
ADDED: Oh, and there is a forum over at the watchguard website which would probably be helpful.
