On YT there are a lot of videos that show how to flatten the sole on hand planes. It seems fairly easy, but I wonder if there are anything that can go wrong if the tecnique is not right?
Can one accidently be "rounding" the sole instead because of more gringding at the ends than in the middle of the sole ?

Is a granite plate from a window usually flat enough for the job ?

What grit size should I end up with ?