How do rhyme schemes work?
It depends on the last word of each line.
Take Twinkle Twinkle Little Star for example
The last word in the first line is star. Star sounds like "are". This sound will be called A. The last word in the second line is are. Are and Star are rhyming words so both of them are called A.
The last word of the third line (high) does not rhyme with 'are' and 'star' so it will be called B. The last word of the fourth line (sky) does not rhyme with star and are but it rhymes with high so it is called B.
So every last word that rhymes has the same letter and every new sound is represented by the next letter in the alphabet. Here's an example:
The people along the sand (A)
All turn and look one way. (B)
They turn their back on the land. (A)
They look at the sea all day. (B)
As long as it takes to pass (C)
A ship keeps raising its hull; (C)
The wetter ground like glass (D)
Reflects a standing gull. (D)
(The poem is Neither Out Far Nor In Deep by Robert Frost)