Install OpenVPN

apt-get install openvpn

Need to generate a key

openvpn --genkey --secret /etc/openvpn/myhost.key

If you using the OpenVPN tap device with IPv6 there is only very little configuration left (/etc/openvpn/myhost.conf):

dev tap
secret myhost.key
script-security 3
up /etc/openvpn/myhost.up

The actual work is done in the UP-script:

#!/bin/sh
ifconfig $dev up
ifconfig $dev add 2001:db8:abba:1000::1/64