This is the link for registering to vote in Texas. This form must be printed out, legibly filled out, signed and mailed to the county of residence of the applicant. The application must be received 30 days prior to an election. This allows time for processing. For Nov 2020 elections, there must be a postmark for October 5 or earlier. Early voting begins Oct 13-30.
For Bexar County the address is
Registrar of Voters
1103 S Frio Ste. 100
San Antonio, TX 78207-9950