After the deregistration
Most of it happens by itself. Three things do not, and they are listed here.
The check costs nothing and you reveal nothing during it.
What happens by itself
Vehicle tax
The registration office reports the deregistration to the customs office. Overpaid tax comes back to the account it was debited from.
Insurance
The registration office informs your insurer, who usually puts the policy on hold without premium.
The register
The entry in the central vehicle register is updated. You do not have to report anything.
What you have to do yourself
Keep the registration certificate
The code on it is needed if you register the vehicle again later. Do not throw it away.
Dispose of the plates
They are invalid. Dispose of them, or keep them as a memento.
Call your insurer
If you want a laid-up policy or want to end the contract entirely.
Do not forget: a deregistered vehicle may no longer stand on public ground. A deregistered car at the roadside can be towed away.