Assuming you are asking how to remove moisture from a refrigerant system that is already installed and in use:
The first step is to identify the source of the moisture. If the system was installed properly, there should not be any moisture present. However, over time, leaks can develop which allow outside air and humidity into the system. These leaks must be repaired in order to prevent further moisture from entering the system.
Once the leaks have been repaired, the next step is to flush the entire system with a dry nitrogen gas. This will remove any remaining moisture from within the system. Finally, new filter-driers need to be installed in order to absorb any future moisture that may enter the system through new leaks.