Don't touch the "/etc/sysconfig/iptables" or "/etc/init.d/iptables" files unless you know what you're doing. You can create a simple script, then save the changes to the appropriate files.
The script can be placed anywhere on your system as a standard file, then do "chmod +x scriptname" to make it executeable.
It can be executed by "./scriptname" or "/directory/names/scriptname".. You can't simply type "scriptname" if you're in the same directory.
The variables are used inside the script where you would want to use a certain value more than once, so if you used "eth0" a few times your script might look like:
# EXAMPLE ONLY
iptables -A INPUT -i $INT_DEV -j LOG
iptables -A INPUT -i $INT_DEV -j ACCEPT
This just saves you having to type "eth0" in all of your commands. It doesn't matter which may you do it, however using variables and assigning values allows the script to be adjusted easily if you make any changes to your networing configuration.
Remember, bacis shell scripting is just adding a bunch of commands into a file that you can simply type at the command prompt one after the other, it just automates it.