How to configure multi wan tier preference for Squid

[siddharthm]

Hi
pfSense - 2.1.5
Squid 2.x
Squidguard

Lan 1
Wan - only for incoming
OPT1 - Main internet
Opt2 - Backup internet

I want to understand what rule should be use for squid to use OPT1 as primary outbound gateway and Opt 2 as backup.

Thanks