Practice makes perfect. I used to take scrap metal and make sculptures. Test different sizes of metal and voltages and wire feed speeds. Go in C strokes.. Just up and down up and down. A perfect weld will look like a roll of nickels. You'll be able to hear the consistency of the weld too.. If it pops and cracks a lot.. You need more gas, if it just makes a burning sound... you're burning holes in it. Practice it. Hell, I'd rather stick weld.. but It's kind of hard to do with a car that can only be jacked up so high and it is harder to remain consistent without a ton of practice. More basic.
Take it to some small ass town shop. They'll charge you 30 bucks to do all of the welding. Ask him if you can watch.