Car seat travel trays usually include a strap and buckle system. To install it, you should release the buckle, place the tray on your child's lap, take the strap and guide it around the seat and buckle it up on the other side of the tray. Here is a video representation that may help you visualize the process better. There is also a type of travel car seat trays that you can attach to the seat in front. You guide the belt around the front seat and buckle it up; the tray stands independently of your child like a small table in front of them. However, I think it is better to lay the tray on your child's lap because there's always the risk of your child hitting his head in the tray if you stop the car suddenly or not being able to reach something if it's attached to the front seat.