Essentially, it's as follows:
Create manual outbound NAT rules from each private network to each public network, respectively
Add load balancer pools for each multi-WAN variation you want. In my case, I have one where both are round-robin, another where WAN1 fails to WAN2, and a 3rd where WAN2 fails to WAN1
Add outbound firewall rules for your local segment(s), choosing the appropriate gateway to be sent out (e.g. Round Robin, WAN1->WAN2, or WAN2->WAN1).
If that's not quite specific enough, have a look at the multi-WAN wiki entry: http://doc.pfsense.org/index.php/MultiWanVersion1.2