It will depend on what is already in the mill.
Depending on if it has a motion controller and/or, there may already be a suitable analogue output for controlling spindle speed.


However, essentially that board needs a 24V power supply, and a step output (it claims PWM, but I'm pretty sure Mach3 using parallel port doesn't have any PWM outs, it's all step/dir). It then converts the step output to an analogue voltage, which is fed into the VFD.


The other option would be to use Modbus to control the VFD, but even for the IT/electronic literate, it can be a challenging option to implement.