Pull up the barrier and one of the car park's team will speak to you through the intercom. You'll be asked for your booking reference, then the barrier will lift to let you through. Then just park your car - you'll be told which lane to park in - and wait for the transfer bus to the terminal. Don't put your credit or debit card into the barrier - that instruction is for customers who've not booked and paid for their parking already. You don't want to be charged twice.
The transfer bus will pick you up from the terminal, where you were dropped off. Just collect your car and head to the exit barrier. A member of staff will lift the barrier for you. Again, don't put your credit or debit card into the barrier.