Some syllables start with two written consonants. Sometimes they blend into ONE syllable (กล in กลัว glua — a true cluster), and sometimes an unwritten short "a" slips between them, making TWO syllables (สนุก sà-nùk). Either way, the rule that matters is one you now know how to use: the FIRST letter of the pair is the one that runs the tone rules.
True clusters blend both consonants into one syllable — the second letter is always ร, ล or ว. The first letter's class sets the tone: กรอบ gràwp is low because ก is mid class and the syllable is deadi; แผล phlǎae is rising because ผ is high class; พร้อม phráwm is high because พ is low class with mái thoo. These are the common ones:
A few written clusters are fakes — the ร is silent, or ทร reads as s. Only a handful of words do this; meet the two you'll see most:
When the pair DOESN'T blend, an unwritten short "a" appears after the first letter, making two syllables. The first letter still runs the show: if the second letter is a group-2 sonoranti, the first letter's class sets the tone of the SECOND syllable. That's why สนุก is sà-nùk (low tone from high-class ส) and ขนม is khà-nǒm (rising from high-class ข) — the silent-ห trick from the finals step is this same idea in disguise: