how to achieve 0,2 or any other surface finish roughness on CNC MILL?

Different machining operations typically produce specific surface roughness ranges. Refer to the attached chart for machining operations that produce a 0.2 roughness.

Select a machining operation in your CAM software that is capable of producing the particular surface roughness. Since that operation has a range of possible roughness values it becomes the responsibility of the machinist to produce the specific roughness.

Step-over, feed rates, chip load, etc. will affect surface finish. You will need to work closely with the machinist when setting these values. Don’t expect to just set up these values and produce g-code to produce the desire surface roughness without consulting the machinist.

and don't forget to clearify which type of roughness is meant
- Ra
- Rz
- ...
this should be identifyable at your drawing (hopefully)

A few things to consider. Is the surface finish esthetic or functional? If it is esthetical, a surface roughness gauge with a range of 0.2 Micrometer should be sufficient. If it is functional, depending on the requested certification, a certified calibrated surface roughness gauge with a range of 0.2 Micrometer should cover it, otherwise a Surface-Roughness Tester is required.
In most cases, to test the requested surface finish is to have the proper surface gauge on hand. Test surfaces after using your available equipment, cutting tools, surface speed/feed, and lubricants. Why such approach is required, because a $10K piece of equipment does not yield the same results as a $100K worth, and the same also applies to the cutting tools.
Raw material types, area sizes, and the type of surface cutting plays a big role on surface finishes. When in doubt, do surface grinding. When the “proper” equipment or tooling not available, do the surface cutting with sand paper or lapping tools as a secondary option.