Yeah but did you buy the gun harry, those can be pricey and extra mixing tips if needed add up too.
What are you having problems with? Its not too hard really. First make sure the replacement piece fits well and then grind the mating areas. Squeeze an inch or so out of the gun and mixing tip onto a piece of cardboard or something and throw that away, before applying to your mating surfaces. (need to do this so you are using well mixed material on your piece) Apply a good bead and clamp in several places the piece into place while the adhesive cures, or if you can't get clamps screw it into place. The clamping gives it strength. As long as you applied a decent bead and it squeezes to cover the seam, the seam should be protected by the adhesive. Grind or sand your patch area down to metal and you can do normal filler work to finish it off, or better yet spray epoxy primer on the area and do your filler work over that.
Don't forget to spray protection on the backside of the repair if you have bare metal. I think even if you sprayed epoxy primer on the pieces and roughed it up where you were applying adhesive it would adhere fine also if used where you will have little access after patched to the backside. There are many different adhesive and what they are used for may vary, but generally the procedures are similar but cure and working times may be different. Go here for some procedures of working with panel adhesives. Or maybe look at 3m's site and maybe they have similar instructions.