If you're actually interested in iptables the old packet filter how-to is great:
https://www.netfilter.org/documentation/HOWTO/packet-filteri...
But iptables is turning into just a legacy interface for nftables in modern Linux. See eg:
https://wiki.debian.org/nftables
https://wiki.nftables.org/wiki-nftables/index.php/Main_Page