how to make cnc milling?

As per Rick, but ideally also zero backlash screws on the axes too.
rickharris6 years ago
This is a big question

You need a mill suitable for conversion. If you intend to mill metal it is unlikely you can make something rigid enough.

You need stepper motors

You need a controller system to drive them that understands the output format of your CAD system

You need a CAD system

None of this is impossible and I suggest you google DIY CNC to find out what and how others have approached this.