1. Navigate to Firewall | NAT.
2. Click on the NPt tab.

3. Click on one of the Add buttons to add a new entry.
4. Choose the appropriate interface in the Interface drop-down menu (you can usually leave this set to WAN):

5. In the first Address text field, enter the internal IPv6 prefix, and select the correct CIDR in the corresponding drop-down menu.
6. In the second Address edit box, enter the external IPv6 prefix, and select the correct CIDR in the corresponding drop-down menu.
7. Enter a brief description in the Description text field.
8. When you are done making changes, click on the Save button.
9. Click on the Apply Changes button.