What are the steps of the correct shrinking of tubing?

Before choosing correct tubing you should know the exact diameter of the surface you want to insulate. If the tube has shrinkage ratio 2:1 you need to follow the rule 80:20, which means that the tube should be shrunk by no more than 80% and no less than 20% from its nominal range. In other words, the inner diameter of the tube before shrinkage should be 10-20% more than the diameter of the surface, and after shrinkage- 10-20% less than the diameter of the surface. If the reality will match these calculations you will have a good fitment of the tubing in the result.