Before a registration number can be put onto a vehicle you will need either a V778 retention certificate or a V750 certificate of entitlement. You will also need the 11 digit reference number that can be found on the latest V5C registration certificate.
If the vehicle that you want to put the registration number onto already has a personalised number then you will need to take the number off beforehand otherwise you could lose the rights to use it.
You will not be able to put a registration number onto a 'Q' registered vehicle or make the vehicle look newer than it is.
For further information, please visit the DVLA page 'Put a registration number on a vehicle'.