The old way was to do an iptables-save each time an update to iptables rules is made and then iptables-restore during system startup. New way is to use iptables-persistent which takes care of both of these.
Install iptables-persist using deb package/apt or tool of your choice on the Linux of your choice. On debian during install it will ask to save currently active ipv4 and ipv6 rules and setup necessary restore processes. That’s all!