In "Ode to the West Wind," Percy Bysshe Shelley forms the poem in groupings of tercets, which are three lines of a poem grouped together. (I'm not sure which two tercets you wanted to examine, so we will start with the first stanza.)
Whenever considering rhyme scheme, begin at the first line and examine the words that end each subsequent line. Rhyme scheme uses the alphabet to denote which lines rhyme with each other. For example, in an ABAB rhyme scheme, the reader can deduce that the first and third lines rhyme with each other, and the second and fourth lines rhyme with each other.
The first line of "Ode to the West Wind" is:
"O WILD West Wind, thou breath of Autumn's being."
The reader would mark "being" as the last word of that line (We will mark it 'A').
On the following line, the last word of the line is "dead;" because "dead" does not rhyme with "being," we will mark it 'B'.
On the third line of the tercet, the last word is "fleeing;" this word does rhyme with "being," so we will mark it 'A'.
The rhyme scheme of the first tercet is ABA.
In the second tercet, Shelley uses the end words: "red...thou...bed in lines four, five, and six. "Red" does not rhyme with "being," but it does rhyme with "dead," so we will mark it 'B'. "Thou" is marked as 'C,' because it rhymes with neither of the first two words. "Bed" is marked as 'B,' because it too rhymes with "dead" from the first tercet.
The rhyme scheme of the second tercet is BCB.
This poem's rhyme scheme definitely has a pattern to it. Within each tercet, the first and third lines always rhyme, and the middle line of the tercet rhymes with the first and third lines of the previous tercet.