Treat the 20A outlet as a starting point
The outlet shape alone does not prove the breaker, conductor size or shared load. Forney's material says the welder should be the only load on the circuit and recommends a 30A breaker.
Do not bypass the plug or use an undersized extension cord. Have a licensed electrician verify the panel and install any new circuit.
Prepare the flux-core wire path
Check the 0.030-inch drive-roll side, contact tip, liner and polarity before feeding wire. Keep the gun liner straight enough to feed smoothly and replace damaged consumables.
Unplug the machine before service. Confirm the trigger is released and the wire is secured before changing the spool.
Plan for fume, spatter and fire
Open the garage and use local exhaust near the plume. Move cardboard, solvents, paint, fuel and other combustibles away from the arc.
Wear a suitable helmet, gloves, protective clothing and footwear. Keep a fire extinguisher within reach and watch the area after welding stops.