Okay, first question from me so go easy.

The machine is a Mill and I am controlling it using Mach3. The machine was built by a friend of mine and works well but it has been transplanted to my workshop now and we both agree that controlling the spindle using Mach3 would be a "good thing".

The three phase motor is presently controlled by a Siemens VFD and I have bought this board to modulate the voltage to the controller: https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
It is described as:
Board PWM, cm-112 ···
PWM signal is converted into voltage in DC Mach ··· ···
··· Conversion --->~PWM Duty Cycle 0 100% Voltage selectable. DC 0~10V

So, what I need now are the bits that go between the computer and this board, and the knowledge to connect them together. I need a breakout board and then can I connect this breakout board to the secondary parallel port on the computer? Or what?

Yes, you are talking to a total virgin here. But I am determined to learn and I am going to start here.