Just search the internet for any kind of template that has the general info you need (date, amount, seller name/address, buyer name/address, etc). Be sure to have 2 copies to fill out or be able to give them a copy.
From the DMV:
https://www.dmv.org/tx-texas/bill-of-sale.phpAlso some forms from the state you will need:
https://www.txdmv.gov/motorists/buying-or-selling-a-vehicleAlso, be sure you do the vehicle transfer notification the day you sell the vehicle. It lets the state know the vehicle has been sold, so if the buyer gets parking tickets, toll violations, etc, it won't be on you.