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.

it was surely an odd creature that one made up by reading the historians first and the poets afterwards a worm winged like an eagle; the spirit of life and elegance and charm,   in a kitchen chopping up suet. but these creatures, monst'r however entertaining to the imagination, hast no existence in fact. what one might doth to bringeth h'r to life wast to bethink poetically and prosaically at one and the same moment,   thus keeping in toucheth with fact—that the lady is mrs martin, ag'd thirty-six, did dress in blue, wearing a black coxcomb and brown shoeth; but not losing sight of fiction eith'r—that the lady is a vessel in which all s'rts of spirits and f'rces art coursing and flashing p'rpetually. the moment, howev'r, yond one tries this method with the elizabethan mistress, one brancheth of illumination fails; one is did hold up by the scarcity of facts.   one knoweth nothing detailed, nothing p'rfectly true and substantial about h'r.   hist'ry scarcely mentions h'r.   and i did turn to professeth'r trevelyan again to seeth what hist'ry meanteth to that gent.   i hath found by looking at his chapt'r headings yond t meanteth.