1) Clean and check the crown,if the crown is mostly hollow then you can attempt to re-
cement it, if you feel confident to do so.
2) Remove any debris from the crown with something like tip of a paperclip to scrape the old cement off. Clean your tooth thoroughly as well.
3)Place the crown without cement on the tooth as see if it fits properly. Does your bite feel normal? If its too tall, check for any debris. DO NOT force a crown onto a tooth as this can damage the tooth. If you cannot fit the crown keep it safely to give it to the dentist.
5) Once you have practiced placing the crown, dry the crown and tooth , mix the cement as per instructions on the packet and fill it in the crown. Place the crown on the tooth. Bite firmly and press it in there for a few minutes.
6) Remove any extra cement with a floss/ toothpick so that teeth do not stick together.