When putting a joint out carefully, the most important part is the cherry. The cherry of the joint is the section where it’s lit before it turns to ash and falls off.
There are several routes you can take to put out your joint without destroying it. The easiest method is by suffocating the joint, cutting off the air supply to the cherry. This method works best with a glass doob tube, but you can also use sand. Avoid using plastic doob tubes, because a still-warm joint can melt the plastic.