The warmer has a long square end opposite its jaws. The long end goes into the flame and heat is transmitted slowly to the roller table, and to the shellac, at the other end.
When it is just right the shellac flows. The roller jewel is moved up and down in the hole, with tweezers, to coat it, made perfectly straight and true, then allowed to cool. It is a bit tricky keeping the temperature right. Too hot and the shellac will scorch and be ruined. Also it is critical that the jewel be situated exactly right. It is usually necessary to soften the shellac and adjust the jewel 2 or 3 times.
But this watch also need a new balance staff.
Another step is using the truing calipers to make sure the balance wheel is perfectly flat and round. The hairspring is then installed and it's all set.