It's been a while since I configure a firewall for a new office setup. So here's just a review for myself as well.
The setup of firewall is still the same process for last couple of years, maybe for the last 5-10 years. The only different in my humble opinion is they improve the GUI management interface instead of using the telnet program through the console. You configure the internal IP address e.g 192.168.1.1 255.255.255.0 for your firewall, external IP address for the external interface
and set the routing in the firewall. Where the route should point to your router IP address, not the external IP of the firewall. This is a common mistake when setting up a new firewall.
For route example: IP address 0.0.0.0 point to router IP 126.96.36.199 as your gateway.
Create a test policy:
Please Note: You should create a GROUP for your service to manage allowed ports to get out of your network. Same approach for incoming traffic, you create a GROUP and add the ports you allow to get in to your network.
You can also use your firewall as DHCP Server. I recommend this to entrepreneurs and small business for them to save money for the time being.