You can fill out a voter registration card HERE and download here, but you MUST mail it in. Texas is one of the few states that doesn't allow online registration.
If you contact a Deputy Voter Registrar, (DVR) as long as they register you by midnight on Oct. 5, they've got til 5pm the next day to get your voter registration in.
We're working on a list of DVRs and will hopefully get them posted soon.