How do I fill the Truma Therme properly?

Close the drain and aeration valves (4) completely.

With premixers
Set the control button (2) of the premixer valve all the way to the red symbol (hot).

With mixer tap(s)
Open the hot water tap of the hot/cold water mixer tap or set the mixer tap to “warm”.

Leave the fitting(s) open until the Therme has been filled by displacing the air and water is flowing.