Drive to Beauvais Tillé airport and follow the signs for P1. At the car park entrance enter the 6 digit PIN code, labelled" Car Park Reference" which you received by email when you made your booking. Collect the ticket and keep it safe (you will need it to leave the car park on return). Find yourself a parking space, park up, lock your car and head for P1 which you'll see opposite the car park just minutes walk.
If you don't have a digicode, just pull a ticket from the machine at the barrier. Keep it safe as you will have to show it when you return.