What about spellcheck? If you'd used that you'd see that excellent is spelled with two ls not one. Not only that excellent is an adjective, adjectives describe nouns, write is a verb, you need an adverb to describe it.
How do I write excellently?
would be the correct form, but I don't like the word much. You'd be better off writing How do I write well?
So use spell check, and adverbs/adjectives correctly.
Most importantly read good writers, see how they do it, and some of their skill should rub off.