route cisco sg300 traffic through Watchguard firewall

route cisco sg300 traffic through Watchguard firewall

I need help in determining what I need to research. I have two cisco SG300
switches (sg300-52 & sg300-28) and a Watchguard firewall (XTM-23). The
internet connection currently passes through our watchguard firewall prior
to going into switch A. Switch B is connected to Switch A via GBIC fiber
port. The internet is currently a cable line so there is no need to pass
it through the switch first.
We are in the process of having fiber dropped to the building for internet
though. this presents a problem I am not sure how to resolve and it is
what I need assistance with.
I want the internet fiber to go into the second GBIC fiber port of switch
A, then immediately route all traffic to the watchguard firewall. I want
all the remaining ports from switch A and B to have all traffic pass
through the firewall. Basically, I want the same configuration I have now
without the use of an additional box to convert the fiber to Cat5; I would
like to utilize the second GBIC port if possible.
Can someone please help me by telling me what I need to research as I have
no idea. I do networking out of need and I haven't had the need to do
anything like this previously. If someone wants to help me with
configuration, that would be great, but I am more interested in learning
the process than just editing some configuration files.