The big first step.
The big first step is to finish a basic manuscript. The most important thing in writing a first draft is to get to the end of the plot. What this entails is simply chasing the idea until it gets to its most logical conclusion, and then some kind of resolution. In a previous post I sort of referred to this as 'the wedding,' which is a basic problem in time, speed, distance, and a large number of characters, all of whom have different requirements of their own and are capable of acting independently on their own initiative.
Sight, smell, taste, touch, sound.
The second most important thing is to layer in details of sight, smell, taste, touch, and sound. Heat and warmth are important, in fact my novel 'The Shape-Shifters' was written in winter time, and it has all kinds of winter scenes, both indoors and out. How important is it for a character to stop and put on a coat before leaving the house? That depends on the weather in your book.
Who, what, when, why, where, how?
Everything in the book has to be accounted for in some fashion, or the logic will not hold together, and the reader will just shake their head and walk away, even taking into account, 'the willing suspension of disbelief,' which is integral to the fiction experience, in a book, on film, or any other show.
Any environment has extraneous details that don't necessarily help to develop the plot, the conflict or the characterization. What they might help with is world-building. Our environment has a lot of detail which is almost beneath our consciousness, unless we choose to notice the blue of the sky and the green of the grass, and the brilliant yellow of forsythia or daffodils in spring. Take another look at the world your story happens in. The reader hasn't seen into your brain, into your vision, and so you have to pencil in enough detail so they get the idea.
This works differently in different genres. A historical, multi-generational sweeping saga can go to 300,000 words, and so there is more room.
What kind of room is a given scene happening in? What is the light like in there? What kind of furnishings does it have?
Do some of your characters stand around in a scene while two characters have a long and involved discussion? Give them something to do, even if it's just raise their eyebrows, fart, or interrupt the conversation.
Every so often, I am re-reading something and I wonder if I have reversed or inverted the character names, in other words, used the exact opposite name attached to a sentence of dialogue. There are times to take confusing things out of the book, or anything you have said twice, or repeated in any way.
Fact checking is important. I've checked the spelling on words I thought I knew, and while sometimes the computer spell-checker doesn't actually know how to spell a word, never assume this to be the case--I thought the machine couldn't spell 'embarassment,' but it turned out to have two 'r's,' and by persistently chasing the spelling of that word, I saved myself some 'embarrassment.'
And that's a good thing. If I say an 1896 Mauser takes 7.63 ball ammunition, (a technical term familiar to gunnies,) then I have checked that fact. While many will assume a typo or that I am mistaken; and that in fact it takes 7.65 mm ammunition, the facts are still on my side, and afficionados will appreciate accuracy of fact and statement.
Another case in point. In my new story, 'On the Nature of the Gods,' I use the word 'verklempt.' The first thing I did after putting it in the book was to Google it and checking the spelling and definition.
Let it sit.
The best thing you can do sometimes is to let it sit. Think of any objections that a reader or critic might have, and try to answer them or even just explain things in your head. There's no doubt the writer must know more about the world he or she has built than the reader. Not all of that gets in the story. But you should be able to account for the things in the story in some way, or why did you put it in? Again, there is a time to take things out, if they don't advance the story in some way.
I often have the urge to add things in to a novel. While a couple of good lines might not hurt, at some point you should save them for another time...and put them in your next book or story.