My method:
1) By True-M prepaid hello SIM in a 7-Eleven.
2) Download the True iservice app an Play store.
3) Make sure to modify login settings so you login with fingerprints. *
4) Use the app to buy any package you want, top up, buy validity, etc... **
5) Enjoy
* Make sure you choose the option that enables you to login with your fingerprints. Otherwise the system is gonna send you an SMS with a code and if you're abroad (not in Thailand) you might not get it.
** each time to top up you extend the validity of your card so buying validity might not even be something that you need to do. That said. 6 months validity would cost you... 14 THB.