There are three steps to this:
- Tunnel configuration
- Firewall configuration
- Client configuration
Sign up for an account at http://tunnelbroker.net
Once signed up - choose "Create Regular Tunnel"
Enter the IPv4 address of the external interface of your firewall. Note - this must be able to receive and respond to ping from arc.he.net - the webpage will give you an IP address to allow ping for if it can't ping your firewall.