Williamson County Online Filing: http://www.wcad.org/eservices/online-hs Note that you will need to search your own property address to reach the HS icon link, then proceed as instructed.
Travis County PDF Application:
You can file your own Homestead form and save on certain property taxes. No need to pay a service to file for you - these are scams. Finding the link / form is 90% of the service they provide you - at a cost of $75-150! Got it here free. (Seriously, just a few click to find if you know what you are looking for...)