Editing is more than one process and often involves looking at your work through many lenses and zooms. There are three different kinds of editing, Copy, content (LINE), and Developmental.
Check for grammar, punctuation, passive voice vs. active voice, sentence structure, basic readability.
Go through the novel line by line to tweak and transform language, style, clarity, consistency.
Ask how the story fits together, Do the elements make sense? How are the characterization, setting, plot, theme, and conflicts? What can be done to give it that extra little bit of oomph?