Assistance Car Deregistration, Registration, Reregistration??

Hello, short question about the procedure, since this is the first in this scope for me. At the beginning of May I have to do: unsubscribe my car; private sale to a car dealer from the EU country, handover takes place in a place other than my place of residence/place of registration. Register the car of another person on the same day or later and then register for me (mumbling). How is it best to proceed here now? When do I cancel the insurance of my current vehicle? Automatically with Registration of the new vehicle? When do I complete the insurance for the vehicle which I take over? In advance, afterward? What do I need for the registration of my vehicle at the registration office? License plate, what else? When do I make the registration? After handing over to the buyer / before handing over to the buyer? How do I get to the delivery place with “deregistered” vehicle? 3/4 tag marks? What does the buyer from the EU country get from me? f (which of them, which one in the original, which one not?) What do I need for the registration / re-registration of the other vehicle? Authorization of the person? Which documents still? Where do I get the green plaque? Registration office? Thank you!! Further questions will surely still arise!!