practice practice practice .Hey biggun thanks for smacking me. I didn't weld the rails yet but I practiced on a few scrap pieces of metal, to my surprise I had no blowouts and the welds withstood many blows from a hammer.
If you can hold the tip off the metal an aquease a trigger an count to 4 or 5 any one can do this.
make sure there is no oil , rust , paint or burs on the two parts .
If you do get off center or have issues like pits or a blow out stop sand it smooth an if need grind out the weld . resist to urge to just Keep globing on more weld. better to at least sand it mooth an start over to fill a unfinished weld or fill a pit.
starting in the center of the hole is key as you want to heat that metal up first befreo getting to the sharp edge of the hole that melts the easiest .