The Verdict: Need More Info
Section 11 waives the residence and language tests for the child or spouse of a naturalised Thai. It turns on your relative having been naturalised, not Thai by birth: find the Royal Gazette entry for their naturalisation.
That waiver removes the need for permanent residence, the biggest obstacle in the Thai system. You still need to be an adult under Thai law and your own, of good behaviour and in a regular occupation, and to pass the scored assessment. The grant is discretionary.
A relative who is Thai by birth, however late their papers were issued, is not naturalised, and section 11 does not apply through them. If your parent was Thai when you were born, see our descent quiz; if your spouse is Thai by birth, see our marriage quiz.
Apply through the Special Branch Police with the Gazette entry. If you are under 20, section 12 may let you join a parent's own naturalisation application instead.