Apply a very high voltage between two plates.
If in dry air/nitrogen, 20,000 volts per cm should generate plasma. You can run the voltage up and down to get corona, arcs, or sparks.
Or you can just buy (a) a plasma ball, or (b) a plasma furnace.
Depends on what you are trying to do with the plasma. I once say a plasma incinerator at a high school science fair - the boy made it in the garage; his parents had no idea what he was doing! It was pretty impressive.
Also very dangerous, but he was very careful, and followed good design principles.