After you install the necessary Network Interface Cards follow this simple tutorial.
Edit your Network Configuration and create one new interface to act as your first bridge interface like br0 and assign a static IP address to this interface for management.
Create another network interface or use existing eth<interface> and set its type to BRIDGE, then make sure the 'Bridge Interface' text matches the text you entered on the matching bridge interface br0. This example uses eth2 as the WAN interface (which matches the br0 configuration)
Now you have to edit or create an additional hardware interface in order to bridge it with eth2. In this example the hardware interface will be eth3 and it will also be bridged with br0
Save and Commit your configurations and reboot the server