Here is a quick tutorial on how to add an ingress firewall rule on your CentOS 6 machine.  In this example, we will be forwarding port 443 for HTTPS.

1. Open up terminal if you are on the GUI version of CentOS 6
   
2. Execute the following command
   1. system-config-firewall-tui
      
      
3. Use your arrow keys to select Customize and hit enter
   
4. Use your arrow keys to select which service you would like to allow.  Hit the spacebar to enable or disable the rule and then select Close once you have enabled/disabled the rules you wish.
   1. In this case, I arrowed down to HTTPS and hit the spacebar.
      
5. Select OK
   
   
6. Select Yes