Drive to Beauvais Tillé airport and follow the signs P3. At the car entrance park enter the 6 digit PIN code, "labelled " car park reference " you've received by email when you made your booking. Collect the ticket (you will need it to leave the car park on return). Park in the P3 and just walk to the Terminal.
In case you do not have a digicode, just draw a ticket and keep it as you will have to show it on your date of return. Park your car in P3 and walk to the terminal.