From there, you move into constructing great stories, and constructing great stories is more important than constructing great sentences.
Please, allow me to elaborate.
Okay, let's say, you're just moving along, living your life, when life happens.
In this particular case, what I mean by when life happens is not when life interferes with your schedule.
What I mean is when life happens as a writer.
Suddenly, quite unexpectedly and quite miraculously, an idea for a great story just drops in your head, and you scramble, you do everything in your power to write it down, as quickly as you can!
Next, you write an outline, a road map of your story.
Now, you're ready to write the story, in novel form.
This is where good grammar matters, because it you don't have it, it becomes a distraction and can destroy your story.
What do I mean by distraction?
If you don't have at least good grammar, bad grammar will slow you down, will keep you from writing your story.
The very last thing you want to be thinking about is grammar when you're trying to write what could quite possibly be the next Great American Novel.
What you need to be concentrating on is story, not grammar.
How good should your grammar skills be? Good enough that when you're writing that Great American Novel your grammar skills are in autopilot, or have become second nature to you.
Your grammar skills do not have to be perfect or as good as a professional editor in a publishing house. It just has to be good enough to get you through the first draft of your novel.
There is a reason why they call it a first draft. You can get the grammar right on the second and thirds drafts. The important thing is that your story gets written.
Story first, sound grammar last.
I like to use alliteration when I write prose, but it is not something I am thinking about until I believe I have written a solid story first. Think of things like alliteration as part of the finishing touches you put on your story. Alliteration has nothing to do with the story but it makes for a story that flows or reads like poetry. But remember. You're not writing poetry. You're writing prose.
Write a great story, and I bet, in a perfect world, an editor won't care about the grammar. Even an editor knows you can fix the grammar later.