How to become a journal editor?
Open positions in journal editorial boards are frequently not advertised (though that happens). Instead, when a vacancy appears, the chief editor will typically look for people with the required expertise and ask them whether they would be willing to serve.

Often, editors will have different specialties. So an editor who is stepping down would need to be replaced in his specific area of expertise. Given that he is an expert and should have a good overview of his specific field, he will most likely be asked to suggest possible replacements.

Chief editors will usually be "elevated" from among the non-chief editors, since these already know the journal and its specifics.



So, how to go about to be invited as an editor? Two things:


Build a reputation as an expert in a specific field. Submit good quality papers to the journal you are most interested in (and others), go to conferences etc.
Build a reputation as dependable and articulate. Hand in reviews promptly, and write good reviews. Editors remember people who write good reviews.


Of course, this is not a guarantee of being asked for an editorship.