You will make progress towards this by just playing the game naturally. You need to have at least 10,000 products get checked out at the register. Progress towards this achievement will come by either you checking out customers, your employees checking out customers, or customers using self-checkout. You can check your progress towards this achievement on the Manager Blackboard under the Statistics section. On "Total Products Checked Out" hits 10,000 you are good.
This achievement will unlock after you have closed your store for the night after you have checked out your 10,000th product.