In a word, Practice. We all did "time" in what was refered to as the "grindery" as aprentices, taught to sharpen all sorts of tools including drills, Lathe tools, Milling cutters. A good basis for later life. I can still do the bigger ones, but age(ing) eyesight, has curbed the ability to be accurate with the smaller ones. So now I "cheat" having bought a drill sharpener a few years ago. One "secret" is to have the Stone properly "dressed", if you are going to do it by eye. I still have my drill gauge, we all made them in "training school", which gives the correct angle.
Pete