Each state has different rules and deadlines to register. Most allow registration online, but some do require a paper form to be submitted by mail or in person. The list below provides information for all 50 states.
Additionally, to find an up to date list of states that are allowing voting by mail, click here to see this article from the Washington Post. Not all states that allow voting by mail will automatically mail you a ballot. Those who do NOT live in the following states must fill out a form to receive a ballot in the mail: California, Colorado, Hawaii, New Jersey, Nevada, Oregon, Utah, Vermont, Washington, and the District of Columbia.