Hi! (again). This is the way most editors "become an editor"--they learn on the job and as they develop the skills and experience necessary to be a good editor, their pay increases.
Essential to editing is mastery of The Chicago Manual of Style, an about 800 page tome that contains the standards and short-hand that nearly all book publishing houses use.
You have to learn the editor's marks and short-hand in the Chicago Manual of Style, because that's the editing "mark-up" the book industry uses.
There are different kinds of editing, and publishers will specify what they want done. These are copy editing (also called proof reading by those outside the book biz), line editing, content editing. Real "proofing" or proof reading in book biz lingo is what an author does when he/she gets the "blue lines" or galleys from the printer. It's the author's last chance to correct minor errors before the book goes to press.
Hope this helps! If I have any editing jobs I need to outsource, I'll give you one to cut your teeth on.