Very basic instructions, from what I remember, is that you need to move items out of the storage and onto the shelves in the shop. People will only buy what is displayed.
When a customer is ready to pay - when they wait near the cash desk and they display an icon with money I think - you need to take each item from their basket, scan it using the computer, and then put it on the right side onto the counter. Then the customer pays and leaves.
Again, very basic instructions, the game is unfinished, it was for a game jam and I did not find the time to at least polish it a little and make it playable. 🥲