Inertia for robot: Calculate by hand

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Hi there, just for my understanding. Could you tell me, if I'm right with my thinking: If i want to setup my urdf robots inertias i just have to set the robots inertias main diagonal elements. If my center of mass is anywhere else than in the origin of my link, i change the origin of the inertia. If the mass of my link isn't equally distributed over the link i calculate a new center of mass. Is this correct or do i have to set the other elements of the inertia matrix anytime? If I've put the right main elements in my matrix Gazebo is calculating the other elements by itself. Or isn't that right? Best regard and thanks for your help Max