It is a bit time consuming, but a necessary task if you want a good playing instrument. First you slide this tool over your fret, it is perfectly level and has sandpaper on the bottom. It quickly locates and lowers any frets that are a tiny bit high.
Then you mask off the fretboard and with 800 grit on a sanding block you smooth out the sandpaper cuts visible in this photo. when the fret is perfectly smooth to the fingernail, you polish it with these three rubber blocks impregnated with abrasive dust.
Here are the tools- first a tiny black rubber block with 800 grit sandpaper affixed. Then three grades of rubber polishers. red first, then green then yellow. Now those frets are smooth.
And the final test is done with this "rocker" device. Each surface is perfectly flat. Place it over three frets and try to rock it. If it rocks, the center fret is high. If it does not rock, all frets are level.