Nearly all the information you need is at http://acgov.org/rov/votebymail.htm
There are all kinds of ways to sign up, including permanent vote-by-mail status: online, mail in, call and even fax. I recently renewed my license and was able to sign up for it at the DMV as part of that process. There are two different applications between the one-time and permanent vote-by-mail status, so be sure to choose the one you want.
The ballots will be sent up 29 days before the election, which still leaves plenty of time to sign up. I didn’t vote in the last Presidential election; Clover was sick and I literally forgot it was voting day. I don’t regret it, but I promised myself I would make sure to not hedge my vote on being somewhere on a day four years from then. 