In /etc/modprobe.d/disable_ipv6.conf:

alias net-pf-10 off
#alias ipv6 off
options ipv6 disable=0

In /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6=0
net.ipv6.conf.default.disable_ipv6=0
net.ipv6.conf.lo.disable_ipv6=0

Then run sysctl -p to take effect.

Then in Example Configurations under HE.net Tunnel Details page, in the dropdown menu select Linux-route2, copy all the commands and paste into your server:

modprobe ipv6
ip tunnel add he-ipv6 mode sit remote 1.2.3.4 local 5.6.7.8 ttl 255
ip link set he-ipv6 up
ip addr add 2001:470:18:d18::2/64 dev he-ipv6
ip route add ::/0 dev he-ipv6
ip -f inet6 addr

Test the tunnel:

ping6 irc6.oftc.net