its a funny shaped thing kinda stepped if that makes sense, can't remember the name or part but what you do is let the nozzle sit on the steel and just follow your line or straight edge etc.

The other type called a pepper pot for obvious reasons is a tad more awkward and needs a steady hand and ideally a straight edge so you can guide it along. I made one so the nut rested on it allowing me to keep a constant height and of course a straight line.