Elly is correct. Usually the other map judges will have a discussion about one’s abilities as a mapper and as a moderator before deciding if someone will be able to become a Map Judge.
The main prerequisite to become a Map Judge is your mapmaking ability. Almost all (if not truly all) of the map judges were Mappers first. If you can’t successfully create maps, how can you be one to judge them? All of the judges have made at least one map to prove they can also execute the requirements that players are given.
A lot of map judges also have a decent understanding of parkour mechanics and knowledge about jumps. Improving your knowledge about parkour as a player by educating yourself about different styles of jumps, strategies, and techniques will also improve your map building skills.
Lastly, and probably the least important, most judges have a decent foundation of WorldEdit and can use it efficiently. This goes hand in hand with becoming a good builder in general, not just making parkour maps.
Hope this helps give you some insight