Step 1: Logon to your WatchGuard (Guessing you know how to configure a WatchGuard)
and go to Policy Manager
Step 2:Next go to "Network" in the top tab on Policy Manager and open Configuration from the drop down menu.
Step 3:Go to interfaces tab and choose an unused connection and then select Configure in the top right hand corner.
Step 4: Next open the IPV4 tab and fill in the Interface Name: call it External for example.
Then from the drop down interface type choose External from the list.
Following this choose the option "Static ip" and fill in the details with an ip address and default gateway provided from the ISP.
Optional Step 5: If you don't have any additional IP's with the internet connection click okay, if you do you have additional ip's go to the Secondary Tab at the top of interface settings. Then click the "Add" button on the far right.
Optional Step 6: Clicking add will make a new box appear.
Type in the ip and click ok. (You have to add each ip address manually) Repeat the process Step 5 and Step 6 to add more ip's.