Milling PCBs requires a perfectly level surface and good results can be hard to achieve. You’ll need design software that can export your trace outlines in the board layout as a dxf file that you can then create a vbit toolpath for with your cam software. What I prefer over milling is to spray paint the board and use a laser to burn the trace outlines away which exposes the copper for a secondary acid etch operation. After that you’ll use your design software to export the drill gcode that will create the drilled holes. Most board layout software will export drill code that uses g81 canned drill cycle gcodes that Marlin and I believe even Grbl don’t even support. This is one of the main reasons why I’m abandoning the atmega based firmware platforms and going back to mach3 which supports all gcode as well as USB locating cameras, and many digitizing probe functions.