There is no one size fits all answer to your question. If you want the Eero as primary then you need ethernet from the ONT to the Eero's WAN port. Then LAN port will then con connect to the G1100 LAN port and the G1100 IP needs to be changed from 192.168.1.1 to 192.168.1.X and the DHCP server has to be turned off. Basically you want the G1100 and the set-top boxes to be on the same subnet as the Eero. Guide and OnDemand will work fine but some users complain that remote DVR on the FIOS app doesn't work.
You can also leave the G1100 as primary and just con connect the Eero to the G1100. This will cause a double NAT and will cause issues with VPNs and online gaming. It really depends on what exactly you are trying to do.