But the process is actually pretty straight forward after you get used to the syntax.
All I'm going to do in this post is show an example firewall rule and reference the site I used for reference.
I think the number 1 common mistake I made was just making an ipfw rule without specifying the number ordering.
This caused the ipfw rule to be added after the allow all rule that was already set.
So I manually added a number that was earlier than the allow rule
The following command will add a rule which will
deny all traffic from a specific IP from coming into my machine via em1
ipfw -q add 64999 deny all from 10.129.62.76 to any via em1
The below will delete that same rule
ipfw -q delete 64999
Here is the reference I used to figure this out