Since most academic essays are required to be formal pieces of writing, it’s important to have your tone match this level of formality. To be on the safe side, buy your academic essay a tuxedo and cummerbund, and don’t let it leave the house without them.
Tone is affected by the words you use and how you use them, as well as by the ideas and information you convey. You know how you write when you’re texting your friends? Yeah, do the opposite of that. (Keep in mind that your tone needs to match your intended audience.)
If you have trouble with all this, you might be tone deaf.
P.S. Check out our more literary definition of tone.