I apply in the first week of January and mark that I want mailed ballots for ALL elections held this calendar year. To vote in a party primary, you must check WHICH party's ballot to send to you.
To fill out the application to vote by mail, you need your Texas Driver's License or your Social Security number. There is also a box to check if you do not have either document. The Election Administrator will now write you a letter to let you know that your application for a mail-in ballot cannot be processed without one of these two ID numbers.
PLUS you need your voter registration certificate to look up the VUID number listed. If you have lost yours, you may also look up your voter registration certificate online:
I send my application for a mail ballot in via certified mail (which is not required but is good for my peace of mind) so that I know when the envelope safely reaches its destination. Then you can use the Mail Ballot Tracker tool (if you live in Harris County) to see if your application was accepted OK.