Don’t write, “He was very happy” when you can write “He was happy”. You think the word “very” adds something. It doesn’t.
You have to be able to organize your argument, at the level of the word, the sentence, the paragraph and the essay itself.
If you simply manage to write in spoken language, you'll be ahead of 95% of writers. And it's so easy to do: just don't let a sentence through unless it's the way you'd say it to a friend.
Writing is easy. All you have to do is cross out the wrong words.