Editing is one of the hardest things that a writer has to do. Having laboured to produce the best text we can, we are often reluctant to take hold of the metaphorical red pen in the form of the Del button and consign parts of our work to the dustbin. Sometimes, that's where they belong but recognising and accepting the fact is always difficult.
An independent editor will instinctively correct badly written phrases, spot clumsy sentences, effortlessly remove clichés, and immediately detect purple prose and overwriting. But for us, they don't look like that. We need to become sensitive to just those critical aspects of writing that during the process, we least want to worry about. So it requires a certain distance.
That's why many writers leave their text for a period of time, sometimes months, before taking on the work of editing. You need to see it with fresh eyes, to react to the text much like a new reader will. After a break, you'll bring to the text just those critical faculties that enable you to improve your writing.
There are different ways to edit a draft of course. For example, you can rewrite each paragraph as you go through, keeping a marker at the point you are currently editing. One advantage of this is that you get a good feel for the new quality compared to the old. You also end up with a complete new draft as soon as you've finished the edit. But the disadvantage is that you have to maintain consistency throughout the edited sections. If you decide to change a character name, you'll need to rework what you've done and possibly introduce more errors.
You could decide simply to annotate the complete draft before attempting to rewrite. The advantage here is that if you decide late on to make major changes, you only need to update the notes. This method gives you more breathing space but it's less immediate. Your extra distance can also leave you more emotionally remote from your writing. Of course, the disadvantage is that you still have to write the new draft and rewriting from correction notes can sometimes feel rather mechanical.
Sometimes you'll be editing for a particular purpose. For example, in one story I was concerned about the strength of the verbs and adjectives. It's easy to undermine your writing with adjectives that understate or overstate, or with insufficiently strong verbs. Bellowing quietly, he slowly rushed away – you get the idea. On one story, I highlighted every verb and every adjective and checked each one. It was tedious but worthwhile and improved the story.
One word of caution. If you ask someone else for editorial help, try to avoid using friends, relatives, or other writers. They may be too soft on the writing, or else try to become too involved. Sometimes though, you may have a family member with just the right level of detachment and support you need. But you do need to retain control and to avoid the distraction of negotiating the content.
Editing is an essential skill for all writers so it is useful to practise occasionally by taking a piece of text and reading it critically. It can be a page from the internet, a paragraph from a novel, something from the newspaper, whatever. The skill is in looking at how the words and sentences work. And in thinking of other ways of saying the same thing. The more you do this, the more skill you bring to editing your own work.