First you need the "iptables-persistent" package. This gives you an init script that loads your settings from /etc/iptables/rules.v4 and rules.v6
When you install it, it wll save your default setings in /etc/iptables.
sudo apt-get install iptables-persistent
Remember that the rules are stored here:
A lot of people ask how does this work because iptables-persistent is NOT any sort of system service directly. It is actually a plugin for the default running "netfilter-persisent".
You can see for yourself by checking the contents of the iptables-persistent package
dpkg -L iptables-persistent
It is a plugin installed under /usr/share/netfilter-persistent/plugins.d/ that is automatically called upon start or restart of "netfilter-persistent" which happens by default during bootup.
So if you installed iptables persistent and wanted to apply your rules just do a "systemctl restart netfilter-persistent" and it would load the new rules from your rules.v4 and rules.v6 iptables config files stored in /etc/iptables/