As a trained welder and I own a welding shop; Flux core is not good for sheet metal and I recommend a 75%/25% gas mixture and a E-70s wire 0.23"
If all you have is flux a couple of things.
1) Practice on other metal of simular thickness.
2) CLEAN! CLEAN! CLEAN! no paint or grease or rust. Flux core on that thin of metal is tough enough let alone with that stuff preventing a good mixture of metals.
3) look at you-tube- there are plenty of videos explaining to a new welder how to make it work. Even videos on welding floors and body panels with flux core.
4) And last but not least- use proper safety gear- safety glasses, helmets gloves and full length cotton shirt if leather is not available.
There is no reason to burn or due damage to your body.
Good luck, get some education and have fun- pm me if you have specific questions and I will try to help the best I can.
Joe
