>>1876106lazy way: just sand it
autistic way: use caliper to measure inner diameter of steerer, find matching dowel, get piece of square wood (like a cut 2x4), drill hole the size dowel in piece of wood, fit sandpaper onto wood, put dowel through wood, put dowel into steerer, spin piece of wood to sand steerer until the amount needed is shaved. This is similar to diy barrel facing for guns, do this if you really care about it being flat, in the end it doesn't matter since you'll be using a spacer or the top cap with the stem extending past the steerer anyway.
