How do you use diction in an essay?

Diction is simply word choice. For example: you have multiple words for "Hello." There is "Hi," "Hello," "Hey," etc... In some cases you say "Hi," when in other cases you might say "Hello." Why choose one word over the other? Well that is diction. It is a conscious choice by the author to use a certain word. Usually the word choice is for emphasis. In cases such as the one I gave however, it is for familiarity. People tend to use different greetings based on a level of acquaintance with the other person.