You need to change 2 things only,you could read a guide..but in any case,change these 2 with your ip
# This is transmitted to the clients connecting from an external network, so it has to be a public IP or resolvable hostname
ExternalHostname=your ip goes here
Open l2.ini with l2fileedit and find srvaddr(or something like that) and put your ip there.Then save it.
You can find your ip here
www.whatismyip.com
Simple and easy.