Catering orders can be for specific dishes or food categories in general. If it's a specific dish, you need to cook the needed amount and fill the box with it. If it's a food category, you need to choose a dish from that category, cook it and fill the box with it. You can only fill the boxes with 1 type of dish, so you'll need the full amount before you can see the "Fill" button on the box.
Each catering order has 3 normal order boxes and 2 bonus boxes. You only need to fill the normal ones to send the truck, but if you fill the bonus boxes, you'll get more rewards in return!
You have a specific amount of time to finish the orders and this time might change according to the order.
Once you get a catering order and complete it or send away the truck empty, a cooldown starts. Currently, you need to wait for 2 hours before a new order arrives. This time might change with the balance changes made to the game by Game Designers.
If you don't want to wait, you can make the next truck arrive immediately using Magic Sauces. Just tap on the catering building to see the order status and speed up the process!
If you have not cooked enough of a dish, you can also ask for help from your club friends! Of course, you would need to belong to a club first. You can check out this article to learn more about cafe clubs. Once you are in a club, you can tap on the small hand icon to ask for help from your club friends!