There are tutorials on this forum and other places too about using a sanding beam which is usually a flat section of metal, aluminium extruded square tube, about 300mm to 450mm long, with some 400 grit wet & dry paper stuck on one face, and gently run it north/south up and down on top of the frets. Where you have marked the high points with a black sharpie or similar marking pen, as you see those marks disappear and reveal shiny metal, job's done and you move on to other areas that need attention. Very important not to focus too much on one section as you risk dropping the level across many frets.

Downside after sanding frets is that you need a crowning tool to re-profile the sanded frets and they are not cheap - $55 plus postage at Real Parts and come in 3 different sizes. Some folk reckon it can be done with small triangular files but that looks way harder than using a proper tool. Also need to remember to mask off the whole fret board to prevent any damage from any accidental slips.