This link takes you to a page to fill in the required information (after verifying that you're a U.S. Citizen and will be 18 years of age on or before Election Day.)   
Fill out the info (there's optional info you don't have fill out- like phone number, gender, etc.)
Then it will take you to the form with the info input. There are instructions- basically, print, sign, date and mail to the Elections Office. 
In Hays County, the address is:
Hays Elections Office
att: Voter Registration
120 Stagecoach TR
San Marcos, TX. 78666.

BE SURE to have it in the mail well before Oct. 5. It MUST be received in the Elections Office by 5pm on Oct. 5.