I've only done it a couple of times but here goes
Go to your system preferences and open the port you want to use in Sharing/Firewall. It may already be set up or you may have to go into new, give it a name and choose the ports you are opening.
While you're in System Preferences, go to Network and check the TCP/IP details for your network connection. It will tell you your IP address (eg 10.0.1.3) and your router (10.0.1.1)
Then go to the Airport Utility, select your airport base, click Configure and go to Port Mapping.
Click on 'new', put in the port you want to open, put in the computer you want to map it to (eg the 10.0.1.3) and the port number you opened in your Sharing.
Update the base settings, your Airport base should restart and you should be done.