SOME TOOLS, SOME THOUGHTS
A few resources for editors, particularly those just out of school or still new to the role, to be added to the many other informational pieces you'll find posted online. Yes, read all you can lay your hands on. Yes, set up informational interviews. Yes, join editing groups and talk with seasoned editors. Yes, take internships or low-paying work to gain experience and samples. All of that helps, as does practice, mentored practice.
Because, as is the case with any learned skill, getting good at editing is not merely about the hours you put in. It's about those hours of "deliberate practice." Guided, focused practice, with correction. (Not Malcolm Gladwell, but Anders Ericsson.)
So, if you are able, above all: find a mentor.