A paddle with tear-away sheets of some sort of fine grit paper. Advance the lead, put at x* angle to the paper, rotate between thumb and first finger to cause rolling motion. Stop when the dart is lethal enough. Same-same for wooden pencils that are not of the flat carpenter style, which generally want to keep the flat lead flat and the tip worked down to a 45*-ish chisel tip.
It's not rocket surgery, folks.
stay safe.