On the flip side, if you loved your job, your coworkers, your boss, the cappuccino machine, everything about the place, don't write page upon page waxing rhapsodic about all that you're giving up by leaving. A brief, heartfelt compliment to the company, or a thank you, is fine.
What Else Should I Do When Resigning? First, out of respect and common courtesy, you would ideally tell your manager in person that you are resigning. If you can't wait for that or work remotely or some other complication prevents that, the telephone is the next best option. You can choose to follow up or accompany that discussion with a formal letter of resignation afterwards, but it would be a rare workplace that would require that formality.