How can you extend a 6-month visa to stay in Thailand for 9 months?

the 6 month METV is good for unlimited entries of 60 days each for it's validity (6 months from the date it's issued)

You get the visa as close to the date you're comin' here as you can so you don't burn down the validity NOT being here 😉

Works like this

enter get 60 days

when that runs out, exit/re-enter the country to get a new 60 days

when that runs out, exit/re-enter the country to get a new 60 days

just before the VISA expires, exit/re-enter the country to get a final 60 days (that takes you to 240 days)

then get the one 30 day extension you're allowed <- taking you to about 270 days or 9 months