To place a crown, your dentist must reduce 1-2 mm of the tooth to make room for it. Your dentist will then use a piece of thread or cord or use a laser to push the gum down around the tooth, to take an impression of the tooth. The impressions are sent to the lab where the crown is made. During that time, you will have a temporary crown. These crowns are usually made of plastic and are made in your dentist's office on the day of your visit. They are not meant to last. If a temporary crown is left in the mouth, the cement eventually washes out, and the tooth can decay.
At a second visit, your dentist will remove the temporary crown and test the permanent one. Sometimes crowns need additional polishing, or glaze or some other adjustment before they are placed. Once the crown is ready, it's cemented to your tooth.