With your sandpaper wrapped around a sanding block (rubber sanding blocks are best and can be purchased in automotive or home repair stores), start sanding the filler using 150-grit sandpaper. Sand lightly and evenly over the entire surface of the repair with broad circular strokes. Sand past the edge of the filler to create a smooth transition. When the filler is pretty close to smooth, switch to the 220-grit paper and continue until it's even. Take your time!
It's not unusual to miss a spot or realize there are some gaps or pits in your filler. If this is the case, mix a new batch of filler and repeat the process until it's smooth. You'll sand away most of the filler, leaving the dent filled and a smooth transition between metal and filler.
