Mach outputs a PWM signal to control spindle speed, so that's 0-5V (assuming 5V parallel port). You therefore need a circuit to convert 0-5V to -10 to 10v. You can use op-amps to subtract 2.5V from the signal (so it's -2.5 to 2.5v), and apply gain of 4 to get the correct range. Use an RC filter to get an analogue voltage level from the PWM signal.

Something like this circuit:
Non-Inverting Op-Amp Level Shifter

The op-amp would need a +-10V supply... one easy way, though probably not the cheapest way to get that is with one of these DC-DC converters.

Or you could get a breakout board which does it for you.

If you're using LinuxCNC then you could control the spindle speed using any of the position control (step/dir, cw/ccw etc) inputs, which is probably easier as it saves making the above circuit.