Sometimes, cavities are too big for permanent fillings to be placed, it is recommended to place a dental crown on the tooth. It is very typical to place a crown on top of a root canaled tooth, but that is not ideal to always to put a crown on root canaled tooth. If small access was made to access the canal and there is solid structure remaining, it would be better to put just a core-buildup (a permanent filling) on the tooth instead of putting a crown. Therefore a consultation by your dentist is highly recommended to either put a crown or core-buildup on top of your root canaled tooth.
During the first visit, your dentist exams to see if the tooth needs a crown. If a crown is indicated, a bite registration impression is taken to make a temporary crown usually. If there is no structure remaining this step may be skipped and go to the next step.
Next step would be preparing (cutting) your tooth to make a space so your crown can seat on the tooth. Depending on the type of crown it may be cut little bit below the gingival margin or at the margin. Typically porcelain fused to metal crowns have little metal margin showing at the gingival margin, so it is the best to prepare the tooth little bit below the margin to hide the metal showing up. If that is esthetic crown such as Zirconia, Procera, eMax, etc the tooth does not have to be cut below the gingival margin because there is no metal showing up.
Once the tooth is prepared, a final impression is taken around the area. Deeper the margin below the gingival line, it may be more difficult to take the impression, so more than one impression may be taken for better accuracy.
The impression could be sent to the lab to fabricate a crown or the crown could be made in-office for the delivery. If the crown is sent out to the lab, you will be wearing a temporary crown for sometime before your dental office gets the permanent crown. A temporary crown is made of not strong material and bonded weakly so it can be easily taken out. It is important to not chew anything hard while wearing the temporary crown. Also, temporary crowns have some bacterial leakage so wearing the temp crown for more than two weeks are not recommended.
Once your dentist has the permanent crown ready, it will be cemented on the tooth. The cement used to bond the permanent tooth is very strong and they are not designed to be taken out so it is important that your dentist check to see if the crown is seating good and if the occlusion is good prior to cementation. Intra oral exam and x-ray, especially bitewing x-ray may be used to check to see if there is any open margin between the crown and the tooth.
References:
https://en.wikipedia.org/wiki/Crown_(dentistry)
https://www.mouthhealthy.org/en/az-topics/c/crowns