You will be asked to take part in a role play session. Look for ways that you can help the customer, try to calm them down, as they will be in a rush, and ask them questions to make sure that you fully understand their needs.
Help them to make good choices about which productes they will buy, and always end with the question "is there anything else that I can help you with?", that alone will earn you brownie points.
At the end of the role play, you will be asked to complete a form about what you did. If you think that you could have done something better, say so, as your commensts will be compared with the assessor's, so be honest with yourself.
Good luck.