Evaluating criteria

Write. You have to practice to get better. If nothing else, write a journal.

Become a connoisseur of bad writing. When you spot something that seems bad, try to figure out why. Once you become good at finding mistakes, you'll be less likely to make them.

Not knowing what to write about is more a problem when writing stories, but essay writing is basically thinking out loud.

When writers' block does strike, pacing around or going out for a walk helps.