Instructions are provided on how to take the impressions of your teeth and I think there is a video link provided as well. They are not hard to do, just make sure to slide the trays straight so you get a good mold. There was enough material provided for a redo because I know I goofed up once.
Everything fit well that I have purchased including top and bottom hawleys, top and bottom stretching plates, and a modified activator. I didn't buy these all at once mind you, they held onto my impressions and I add to my collection from time to time.