More advanced writing requires more, and until I learned what revision really is, and how to do it, my writing didn't progress. Even if a first draft--the real first draft--has no spelling errors, no grammatical blunders, and no sequencing errors, it may not be publishable (or anything you'd want under your name ten years later even if it is published.) Hence, revision.
Revision, I've said before, is actually re-vision...you have to re-see the work, see it as an outsider sees it, see it the way you wanted it to be, and then do what's necessary so that the reader, the outsider, sees what you wanted them to see in the first place. And hears what you want them to hear, as the words run through most brains as a voice--as your voice, the storyteller's voice.
The hardest revisions are those which you have to do immediately after writing the piece, by yourself, in a hurry. It's hard to see the piece clearly if you've just written it, hard to be that stranger coming to the work you know so well, hard to see the gap between what is on the page (or screen) and what you hoped to convey. The easiest are those allowing sufficient time and the help of a good alpha reader.
Let's start with what can go wrong with something written by a writer who is competent in the basics--who, most of the time, doesn't confuse homonyms, doesn't misspell words, doesn't have trouble with subject/verb agreement. Someone who knows what they meant to write (length, topic, tone, basic structure) and knows what belongs to those parameters.
For me, the architectural metaphor works: in building a structure, you can have problems at the design phase (something's in the design that just can't work--a bathroom with no water supply, for instance), in the construction phase (a good design wasn't followed, or sloppy construction left the staircase hanging by one bent nail), and in the "finish" phase (although there's good design and sound construction techniques, there's a heap of construction debris by the front door, half of one wall never got paint, the handles in the kitchen cabinets don't match.)
So every piece of writing intended for publication has a design (whether you make it all first, or work as you go along) and if the design has flaws, those flaws will ruin it despite excellent craftsmanship in the writing: perfect spelling and grammar can't save a lousy plot. On the other hand, good basic designs can be ruined by lack of craftsmanship in execution. Bad writing (which usually means bad craftsmanship in constructing sentences, paragraphs, scenes) is hard to read. And finally, when editors are looking at stories to publish, and readers are comparing writers to follow, there's the finish level...does the craftsmanship of construction continue to the finish, to making an attractive, desirable interface for the reader, the book equivalent of "curb appeal"?
Many people--including some of your friends and relatives--focus immediately on the finish level: misspelled words, typos, the odd grammatical blunder. This is what they were graded down for in school, and this is what they see as the first thing to fix. But it's not. Smoothing and polishing the top layer is the last thing to do...other revision steps may eliminate that misspelled word altogether. More experienced readers, and editors if they want to bother, will see problems at many levels, and hand you a confusion of comments that relate to different levels of revision.
Depending on circumstances, you now have two ways to go: 1) start at the design level, fix all the design flaws, then do to the construction level and fix all construction flaws, and finally do the top level (spell-check, etc.) or 2) start at the front of the book or story, and work through it front to back, sequentially. Both approaches can produce a good final result, but they feel different while you're doing them.