Should be pretty self explanatory, simply print out and add your metal parts! I recommend using some PTFE (aka Teflon--the white parts in the picture) as insulators if you plan to allow continuous arcing for more than a minute or so since the metal will become very hot and begin to melt the printed piece. You can purchase PTFE in many shapes and sizes from McMaster. No need for the insulators if you just plan to run it briefly. Ideally you would use tungsten for the electrodes, but stainless will still last quite a long time.

As always, don't play with high-voltage unless you know what you're doing (but you'll obviously need a lot more than just this to be dangerous.)