It is assumed that you have already created an instance.
1) Networking → Virtual Cloud Networks → <network_name> → CIDR Blocks/Prefixes → Add CIDR Block/IPv6 Prefix → Assign an Oracle allocated IPv6 /64 prefix → Add CIDR Blocks/Prefixes
2) Networking → Virtual Cloud Networks → <network_name> → Subnets → “…” right of the <subnet_name> → Edit → Assign an Oracle allocated IPv6 /64 prefix → enter any hexadecimal number between 00—FF (e.g. 7E) → Save changes
3) Networking → Virtual Cloud Networks → <network_name> → Route Tables → Default Route Table for <network_name> → Add Route Rules
Protocol Version: IPv6
Target Type: Internet Gateway
Destination CIDR Block: ::/0
Target Internet Gateway: Internet Gateway <network_name>
4) Networking → Virtual Cloud Networks → <network_name> → Security Lists → Default Security List for <network_name> → Egress Rules → Add Egress Rules
Destination CIDR: ::/0
IP Protocol: All Protocols
5) Compute → Instances → <instance_name> → Attached VNICs → <vnic_name> → IPv6 Addresses → Assign IPv6 Address → Assign
Testing
Just ping google.com. It will be done over IPv6. If not, try rebooting the machine.
Port opening
As for IPv4, but using ::/0 instead of 0.0.0.0/0 and ip6tables instead of iptables.