Judging is a technical skill. Like anything else, it requires study and practice.
Parents, teachers, supervisors, even police, landlords, waitresses, and pizza delivery guys operate as judges from time to time. The rest of the time, among peers, we don’t judge; we only have an “opinion”.
If we do well in smaller courts of life and have good opinions among our peers eventually, other people will ask us to judge them. Some call this “job promotions”, others call it “getting the girl”, “respect from children”, or “winning elections”.
It starts with being serious in whatever small things we may judge.