To finish the level successfully, you need to complete all of its objectives. You can see them before the levels starts and while playing at the top of the screen.

Guests order dishes that are displayed next to them. Your goal is to cook the dish and give it to the guest. Each guest has a waiting bar that slowly decreases. When it reaches zero, the guest will leave.
Every sold item will raise the bar a bit.

Some dishes can be given away at once, while some will have to be cooked first. Tap the ingredient to start cooking and then tap the finished dish to put it on the plate.
Then tap toppings if you need to add it.
When the dish is ready, tap it to give the order to the guest.

Most dishes will burn if you don't put them on a plate. In addition, you can only decorate dishes and add toppings while they are on the plate. If the dish is burnt or you added a wrong topping, double tap it to throw it away.

You can perform combos in the game - serve several dishes one by one. Each dish in combo after the second brings more coins than it would normally do.
To start a combo, you need to serve two meals in a row, and then a combo action timer will appear. Try to serve up to five meals in a row to get the most out of it.