You add the impression mold kit  to your order and follow the shipping instructions, they'll send it to a lab in China then you should have it back. Follow shipping instructions carefully.

You can order it yourself, but they cannot bear any blame for the consequences of providing it. The quality of your impressions and appliance chosen based on you dental situation will determine the outcome of your expansion correction.

You are still young so the way you guide your development will have to be meticulous. If you are getting something like a Schwarts appliance you can also get some for the lower jaw for additional cost. Be sure to do your upper jaw molds correctly to make sure that the roof of the mouth has the most accurate results because gravity will pull it down moving expansion more towards your teeth than the mid-palate. A common problem with some expanders is that the teeth become tipped outwards, you will have to watch for this.

You will be wearing the appliance for many if not all hours of the day.