It is not easy. Well, it is easy, but it is very time consuming. The felt and paper punchings under the balance rail are what raises and lowers the keys for leveling. The paper punchings are different thicknesses. The felt punchings are thin,medium, or thick, but they likely do not need replacing. If you have a handful that are too low, or too high, adjust the paper shims. The felt punchings should ALWAYS be on top of the paper punchings, but seldom are, due to laziness at the factory.
If some keys are real low, it is possible that the felt punchings are missing ( eaten by mice or moths.)