To forward all email when it arrives (excluding any email explicitly caught by discard rules, or categorised as spam), open the Settings → Rules screen. Under the Forward section, enter the email address you want to forward to and click
Add Rule. Then, click the big
Save Changes button in the lower right of the screen.
If you like, you can change "Always" into a rule to only forward messages that match a certain criteria.
If you just want to forward all mail sent to a particular alias, you can do this by directly setting the target of the alias to the address you wish to forward to.
The main differences between this approach and using a rule are:
If these limitations on virus, backscatter, and spam filtering are OK, then you can use the Advanced → Aliases screen to target aliases at the external address(es). To target multiple addresses (for example, an external primary, Gmail for backup, and your main FastMail account), separate the addresses with a comma.