Unlike PAN, with FortiGate you don't have to upgrade individual unit or syncing. What you need to set up initially is:
1. configure HA on both units as well as hostname, dedicated management interface. You can do it via CLI easily. Make sure the primary's uptime is much longer (more than 5 min difference) than the secondary.
2. configure everything else on the primary unit only.
3. Then hook up networks including HA connections so that the secondary syncs up with the primary. It might take quite long time.
If you want to see the progress of syncing, you need to have a (remote) console access to the secodary as well as CLI to the primary. Then you can check checksum and other HA related commands through the connections.