Simply call them on 01293 223 660 and choose option 1 when you're about 20 minutes from the terminal (which will normally be when you're on the M23).
If you're coming from the M25, please call before you reach the M23. Head towards whichever of the two terminals you are flying from and follow the signs for the Passenger Drop-Off point. The North Terminal drop-off is outside the Sofitel hotel, while the South Terminal area is clearly signposted. Park in the left or right lane and an Airparks chauffeur will find you. If you get lost, just give Airparks a call.