Butt Welding 18 Gage w/MIG?

Try to use .023-.025 wire. It requires less heat. Your gap should not be much wider than the wire you are using. If your gaps are 1/8 (.125) or 3/16 (~.185), you've got roughly 5-8 times too much gap. Get a .045 Norton or .040 Dewalt cutting wheel. Depot has the Norton, Lowes usually Dewalt.

If you are welding outside the CFH setting may be ok. In a stable environment, it may be a bit high. I usually run about 14-16 CFH.

Quick hits. The longer you have the gun activated, the more heat you are transferring, the larger your Heat Affected Zone/Area (HAZ). Usually large HAZ = warping/oil canning. You want to minimize your HAZ.

Voltage usually determines the width of your weld. More voltage, wider bead.

Practice on some scrap piece, like the stuff you cut out if still available. Play with the settings and start with your pieces in the horizontal position to get a feel and settings. It's the easiest position to start with. Once you get that down, turn the piece to the vertical position. Your settings may change slightly.

Generally:
Hissing = too much voltage and/or too little wire speed
Popping = too much wire speed and/or too little voltage

Good luck and have fun learning. It's a blast until you cook something real good. Invest in a welding blanket if you have any seat/foam/door panels nearby. That stuff is not real easy to put out once they get burning... DAMHIK!!! :-D