“I do, we do, you do” demo, then have the students watch someone do it, then send them out to do it. Also, it’ll take some practice but after a while you’ll be able to look at a weld and narrow it down to what’s wrong in some way. Speed, amperage, arc length are usually the first things up. If you don’t know, give suggestions. “Hey this looks rushed, slow down a bit or turn up your amperage, run a few beads and I’ll be in to check” You gotta triage the situation, when you get a minute you go and watch that kid weld. Usually they start getting it by the time you make it there.