connecting potentiometer to cnc shield

What I am doing is using a nema 17 to drive a valve seat cutter, I am needing to drive the motor constantly with a speed adjustment and start stop.

I have a uno and cnc shield set up working, I am wondering how to connect a potentiometer to the shield to adjust speed, connect a trigger button to start and stop the motor and a code to do this, I am really wanting it to be stand alone as it is not a cnc machine.

Kinda new at the cnc shield side of arduino.
Some help would be much appreciated