What is the process for labeling the rhyme scheme of a poem

The rhyme scheme of a poem is labeled using capital letters in alphabetical order. The first line of the poem is labeled with the letter A. Any other lines that end rhyme with the last word of this first line is also labeled A. The next line that does not rhyme with A is labeled B, because it is a new rhyme and B is the next letter in the alphabet.

Let's use Robert Frost's "Nothing Gold Can Stay".

Nature's first green is gold, A

Her hardest hue to hold. A

Her early leaf's a flower; B

But only so an hour. B

Then leaf subsides to leaf. C

So Eden sank to grief, C

So dawn goes down to day. D

Nothing gold can stay. D

The first line of the poem is labeled A. Since gold and hold rhyme, the second line is also labeled A. Flower does not rhyme with hold or gold, so it is labeled with the next letter of the alphabet, B. Hour and flower rhyme, so hour is labeled B too. Leaf does not rhyme with hold or hour, so it is labeled with the next letter of the alphabet, C. Leaf and grief rhyme so it is labeled C as well.