Click on the sweet shelves and then on the jars to fill them. You can't have more than 10 sweets in a jar. When a customer comes, he/she will have a bubble with the order above his/her head. Drag the required amount of the required sweets from the jars to the box. In the bottom left corner you will see how many of each sweet the box contains. When the box content corresponds to the client's order, click on the box to close it and then on the customer to serve him/her the order. If something has gone wrong, you can replace the box with a new one pressing New Box button in the bottom right corner.