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.
Why a Resignation Letter? In these digital times, why should we submit a resignation letter, anyway? A paper trail does seem old_fashioned, but in this case, it's a best practice. It makes your intentions clear, as well as the conditions. You're removing any ambiguity about the end of your time there.