You don't.  I didn't, anyway, yet I am a ferocious editor of my own work and a brilliant proofreader.  This latter quality has been discovered in my Real World job and everything that is written is brought to me;  I invariably find a hundred obvious mistakes that everyone else has missed.  It all boils down to experience.
When I first started writing (I was twelve), it was by hand.  Nothing got edited!  When my typing speed overtook my handwriting, I didn't do much more than correct mistakes with a bit of Tippex.  My first PC  (it was an Amstrad....ahem) meant I could do a weeny bit more editing, but even then, I didn't do much.  I wasn't a proper writer yet and considered that everything I wrote was perfect first time.
HAH!
I turned into a Real Writer when I began the Fleet Quintet.  Working more professionally, using what is now an ancient version of Word, I was far more willing to rip things apart;  to rewrite and rewrite until it worked;  to throw out what didn't work and begin again.  Nowadays, the REAL writing for me begins when I do the Big Edit.  My writing seems - generally - to follow a particular pattern.

• The story is created in my head, then in notes.
• The first draft is an attempt to get the story down and is usually an excruciating process of self-doubt, anxiety, wordlessness, and zero inspiration.
• This "first draft" will consist of several edits, usually rewriting of earlier bits because something later on doesn't work.  So there's still a lot of plotting going on as well as character changes, even name changes.  Sometimes there are changes from third to first person.  Sometimes I restart the whole damn thing from the beginning.  But this is ALL still part of the "first draft."  I'm still getting it down on paper, as it were (or on a computer screen).
• The Big Edit comes when the first draft is finished.  I might have a rest period before I start this.  It usually entails reading the whole story in one go (or several if it's a novel) and making notes but NOT changing anything.  That comes later.  Depending on how well the first draft went, the Big Edit could vary in length.  I've been known to abandon a story altogether and then come back to it and finish it with a flourish.  It happens.
• The Red Edit is when I print out the story and read it VERY closely indeed, armed with a red pen.  This is probably closer to proofreading, though by the time I get to the Red Edit, I've already done a thorough spell and grammar check and would  have proofread it before printing it.

With my current WIP (the Prizewinner), I'm doing a combined Big and Red edit.  Hence Big Red Edit!  When I picked up the novel again a few weeks ago, half of it had already had its "red edit" but with loads of notes.  What I really wanted was to have the notes in red on the page, not on a separate bit of paper.  Instead of redoing that first half (it's a LONG novel), I'm now Red Editing chapter by chapter:

• Read the chapter through thoroughly, correctly everything on the page with red pen that you would normally do on a computer.
• Then make these corrections on the computer.  This gives you a SECOND chance to see your corrections which means you really have to think about what you are doing.

So far, this is going quite quickly.  Sometimes a red pen edit doesn't work when you see it on the screen.  Sometimes I see something on the screen that I missed.  
In the end, I'll end up with a novel that has been thoroughly overhauled, which is what I'm aiming for.

The Big Edit has begun!  And it's begun in red pen, which means I have pages of manuscript covered with little red scribbles.  My red pen leaked everywhere too so my fingers looked as if they are dripping blood.  Hugely appropriate when I consider how much of my novel I've had to cut....
I began this red edit two years ago and abandoned it in disgust.  I thought it was a bad novel and not worth saving.  It was also unbelievably long, coming in at 185K words.  No agent in the universe wants to read something that long unless it's brilliant and going to win the Booker Prize.  Picking up where I left off, however, I've discovered it's not that bad.  In fact, I got so involved in reading some bits of it, I forgot to edit it!
I spent two years working on it, from February 2015, probably the last time I've written anything without being conscious of its length (I'm more controlled these days).  I had no idea it was turning into this massive project!  I thought I was never going to finish it.  I battled with the story.  I drowned in notes.  I rewrote and restructured and rewrote some more.  I wrote more notes, more than I ever had for any story, trying to find the male character's voice, failing miserably.  In fact, it's very possible I have failed on every front when it comes to this novel.  I've referred to it before as That Difficult Novel.  In the spirit of hope and a bright future, I'm now going to call it the Prizewinner.  (I may discuss in another blog what I think is wrong with it - and why it works despite that.)
It was meant to be a brief fairy tale.  It was meant to be sensual, a woman's learning experience, a man tortured by some weird affliction that resulted in immense pain.  He was meant to be demonic, she was meant to be heroic.  It was meant to be about beauty and knowledge.
It's now none of these things.  You may catch a glimpse of Beauty and the Beast in it (hasn't THAT been done to death...) but otherwise those nobel themes have long since been lost.  As for the sensuality:  unless you really know what you're doing, you just end up with porn.  Or, worse still, Mills and Boon type porn.  I didn't want either. I'm going to have to cut about ten dripping wet sex scenes from this novel.  Somehow I'm going to have to learn to express their love and desire and all that romantic stuff in a different way.

One thing you learn in your Big Edit:  nothing is ever as terrible as you think it is.  There's always something salvageable, whether it's the story itself, the plot, or the writing.  My Prizewinner has some good writing.  It has a fantastic twist.  The story is worth reading.  But it takes too long to get going (my usual flaw) and sags in the middle.  However, the end is mindblowing.  Getting the reader to get as far as the end is my challenge!

The picture below is very much what the novel is about:  beauty, nature ... and a lot of water.

I’m not wild about making resolutions.  And promising myself that I’ll go to the gym more often or eat more salads or work harder or be friendlier seems to have the opposite effect.  Setting writing goals seems to work, though.

To prove a point, I’ve just glanced at the first page of my pocket diary and notice that I’ve done NONE of the things I intended for 2019.  I never got my Will done.  I never sorted out my mother’s ashes.  (Though I did make a concerted effort).  I did do my tax return, though.  Or I paid an accountant a small fortune to do it for me.
I began the year by decided to help a friend with their book business and set about turning myself into That Person That Reminds You to Write Reviews for Books.  That was a huge bust.  Everyone thought it was a great idea but no one wrote a single thing.  See me slink away with my tail between my legs…
I also planned to go on holiday last year.  But though I looked and looked and tried to plan things and added up my pennies again and again, I only ever got to Margate for four hours. 
 
However, when it came to writing goals, I achieved everything I set out to do.  And then some.  You only have to read my last blog to realise that I’ve been QUITE busy.  I guess I’ll just stick to writing goals instead of resolutions;  it seems I can handle these far better than Real Life.
 
2020 is going to be the Year of Editing.  My new short story needs to be edited and improved and fixed up and, basically, finished off.  Then I’m going to tackle That Difficult Novel, which I’ve mentioned several times in the last few years.  I began editing it last year (wait, was it the year before??) and couldn’t face it.  So this is the year.  When I’ve done that, I’ll edit the first draft of my new fantasy novel which I wrote in the last year (the second in the series.)  Then I can think about writing the third in the series (though I'm willing to wait to 2021 to begin that one).
 
In professional-looking bullet point form, these are my goals for 2020:

• Edit and finish The Crystal Vision
• Edit, shorten and finish Season of the Falling Sun
• Edit Honeysuckle Rage and the Sacrifice Stone
• Keeping looking for an agent for Honeysuckle Rage and the Everlasting Tree.

 
Naturally I’d be quite happy to drop all of this if an agent decides they like the latter book and asks me to work on it a bit so that they can get it published!  And yes, please, send me anywhere in the world to promote it - I could do with a bloody holiday.