Thanks
Stonefall traps work completely independent and automatically. You order them built and that's it.
A dwarf with mechanics will pick the designated mechanic and build the basic trap which will be a slightly dark grey hue. Then a dwarf (does not need mechanics as far as I know, but often is the one who installed the trap) picks up a random raw rock and loads the trap with it. The trap is displayed in white now and ready to strike once at any non-allied creature entering its square.
Once the trap has been triggered, it needs to be reloaded (that is, a rock needs to mounted into it again). This is done automatically by any dwarf, so you also do not need to worry about it.
Personally, I recommend to use stone fall traps NOT for your outermost defenses because they need to be reloaded after every use - so dwarves will run into any attackers trying to reload the traps. Just a sidenote on use.
Good luck and have fun with traps!
Deathworks