If I had to give just one piece of overriding advice for writing well, it’s this: place yourself into your readers’ shoes. Imagine what they’re thinking while they’re reading your words. Anticipate their questions, their confusion, and their boredom, and then prevent them ever feeling those things.
If you then allowed me to give a second piece of advice, it would be to liken a newly written essay to a block of marble. Somewhere in there is a beautiful sculpture waiting to come out; your job is to remove the extraneous bits that are hiding it. As the writer Antoine de Saint-Exupéry said, “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away.” Often the hardest part of writing is looking at a great sentence you’ve composed, full of well-chosen words, and realizing it contributes nothing other than to show off how flashy you can be. You’ve got to rip those out without mercy.
As you then ran screaming from my attempts to give a third piece of advice, I would shout after you, “When you pile too much into one sentence, it’s easy to get bogged down in trying to untangle its twisty, complicated structure, when the better remedy is so simple: just break it up into two sentences! Matt! Maaaaaatt!”