The deadline to apply for Vote By Mail in the 2018 election is October 30th.
Per a law that went into effect on August 10, 2018, voters who cast their ballots via vote by mail in the 2016 election will automatically be sent vote by mail ballots for the 2018 election. These voters will continue to get ballots by mail for every election unless they tell their respective county clerks in writing that they don't want one.
If a person is signed up to vote by mail, they will not be able to fill out a regular ballot to vote in person. However, they will be able to vote by filling out a provisional ballot when they show up to the polls.
Additional information about voting can be found here.