Some people use a rubber spreader for this. I don't. I gently take some 220 g and wrap it around my finger and sand them applying hardly any pressure trying to make sure my coverage is as even as possible. You can unlevel the frets doing this so be careful.

After making one pass with 220 move to 320 then 400 and 600. Once your done sanding move to 0000 grade steal wool and if you have the mind and equipment to do so, take it to a buffing wheel.