If you are doing a manual landing, all A/P modes should be disengaged. If it is an autoland, you must have HDG and V/S already engaged to activate APP mode. Keep all engaged until you disconnect.
I think I know why he’s sliding off the runway on touchdown after an autoland. When you use APPR, it will not apply any rudder on touchdown, the pilot needs to disengage at touchdown and do this manually. The rudder is to counteract a crosswind component. Without it, the only way the airplane can maintain its heading is by banking, which causes it to go off the runway (and smack the wing/engine on the ground).
Also, he’ll want to go to settings (in-app) > general. In there, make sure auto coordination is disabled.