You’ll see the Cheap Parking entrance clearly signposted at the terminal approach roundabout. Your car’s number-plate will be recognised at the barrier and you’ll be given a ticket. The barrier will go up and you just have to find a space.
Find a safe spot for your ticket before you take the 5 minute walk to Departures. Make sure you keep an eye out for the big, yellow submarine - you can see it from the car park - and once you’ve passed that, you’re almost there.