What are you going to run as a control? Normal way to trigger torch on/off is just like you do it for a router spindle: GCode M03 triggers a relay, M05 turns it off. Relay contacts are put in parallel with torch switch so Either will trigger the torch. Your electronics (breakout board or THC) should have provisions to turn the torch on/off via an isolated switch.
The insertion of M03 and M05 at the right time for plasma into g-Code is a function of the CAM software (POST). It's part of the toolpath definitions along with Lead-in's, pierce height, pierce delay, cut height and feedrates for a given material and tip combination.
Look at SheetCAM as a CAM program for 2D (shape cutting) or 2.5D (pocket milling). It's inexpensive and has lots of features you will need for plasma cutting. It's not just as simple as turning on the torch and cutting steel. Read some of the threads about the tip clogging problems others are having and read and understand what a Torch Height Controller does and if you need one.
It's actually more complex to get good cuts with plasma than with a router or milling machine.
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