@Gmdive I'm not sure there are official directions on how to initially set up an Ethernet backhaul capable Deco set form the start with Ethernet backuaul.  If seems you first follow the setup instructions:

Connect the main/primary Deco to the modem/gateway/ONT

Download the Deco App.

Open the app and follow the instructions on setting up the main Deco and adding the satellites.

Connect together with Ethernet infrastructure that may include network switches.

Supposedly the Ethernet connection will be recognized and automatically switched to Ethernet backhaul.